ENGLISH MYSTERY stories, television's "Upstairs, Downstairs" and the Poldard series have built a whole mythical British kingdom in my head.

I love the bits about the breadfast on the sideboard: kippers, always, and acres of toast. Sherry in the library after "Anyone for tennis." The maids and the valets unpacking for one (and repacking with vast amounts of tissue paper). The sweeping staircases, for the unloved wife to fall down, wafting a long train of silk behind her. The secret tunnel in the basement, behind the wine shelves, leading to the abandoned dock on the river. The midnight meetings in the gloriette. The crystals tinkling in the chandelier of the ballroom. The clink of tea cups in the morning room. Who worries about clanks in the night in a room festooned with ancient gold work embroidery?

For years, life somehow never quite got itself together to provide me with a manor house of my very own, or even a glimpse from behind the velvet rope. The instatiable demands of our modest urban cottage have kept us too poor to go to England.

But this year, something strange and irresponsible came over us under the undue influence of an especially good diced chicken with peanuts in spiced sauce at Trudie Ball's Empress Restaurant. We decided to take the money that we'd planned to use in converting the oil furnace to gas, and blow it all on three weeks in the English country-side.

After three weeks in England, I've decided tht it's certainly cheaper to travel by books and television. No sooner than we told the International Travel Service to reserve our seats on British Airways, the dollar took a tumble. In the end, the trip cost us not only the new furnace but a month or two of fuel bills as well.

Now that we're home, I have a pretty good idea of how to manage better, though it will probably all change in the 10 years it'll take us to get up enough money to go again. But for those of you planning to go this fall -- an ideal time to go -- here are our recommendations on how to see English country houses and castles without ending up in the moat.

"Commended Country Hotels, Guest House and Restaurants," and "Stay at an Inn," are useful British Travel Agency publications. We found the Michelin Great Britain most useful, both for attractions and hotels. But the hotel prices in the 1979 edition were sadly out of date. Other people recommend "The Good Food Guide."

People going this time of year can go cheaper than we did. For instance, British Airways has three budget plans, each exclusive of airfare, but offering seven nights in a hotel with a private bath. For $129 you also get continental breakfast, a four-day bus pass and an Avis car for three days (you pay mileage and petrol). An alternate tour at $199 adds three theater tickets and a nightclub evening. Stepping up is the "London Without Cash Holiday -- $399 for all meals, museum, theater and movie tickets, and an Elizabethan banquet, a nightcap, morning paper -- and postcards, prestamped.

I have no advice to you about changing money. On the recommendation of a foremost economic expert who had lived for years in Britain, we did the standard thing of taking half our money in pound travelers checks and half in dollar travelers checks: The first thing we found was that some small village restaurants won't accept even pound travelers checks.

The dollar was going down, down,so in a panic we thought we'd better convert our dollar checks to pounds even though we hadn't yet used all our pound checks. Having decided everything was too expensive to buy (the Harris tweed jacket our daughter wanted did indeed turn out to be cheaper in Washington), we converted our pound checks back to dollars in Washington -- and lost again because in the meantime, the pound was lower than we'd paid in England.

We had much better luck in planning what to see.But then we know more about houses than we do about money.

Reading Nigel Nicholson's "National Trust Book of Great House of Britain" (godine) was a good place to start. But we soon realized that by no means all the wonderful British houses are owned by the National Trust. (Nor are all the houses in his book.) A surprising number still belong to their private owners, often scions of the original families. Leeds Castle in the south is now owned by a foundation which sponsors medical seminars. Warwick Castle is owned by Madame Tussaud's, the wax work folks.

Most are listed in a useful paperbound book, magazine-sized "Historic Houses, Castles & Gardens in Great Britain and Ireland," by ABC Historic Publications. The listings give a good description of just where the houses are and even give the available bus tours to them.

Some 2,000 sights are listed in the compact Automobile Association "Stately Homes, Museums, Castles & Gardens in Britain". The AA guide gives the owner's name and the telephone number -- a good idea, providing you can figure out the complicated English telephone code number system. Both books have maps spotting the attractions. Be careful to note the hours and days. Some places are not open on Mondays. Or Thursdays. Or mornings. Or when the queen is in residence (Windsor). Many are closed in the winter. May to October are the prime times.

The open-to-View card costs $12 in this country, from the British Travel Book-shop at 680 Fifth Avenue, and 7 pounds in England, from the British Tourist Authority 64 St. James St., London, WC1V6HB. (Take your passport if you're buying in London). The ticket admits you to National Trust Houses and some of the more important privately-owned attractions, as well as monuments such as Stonehenge. It is arranged by county.

You can also buy a cheaper ticket only good for "The Magnificent Seven", not the movie but an association of great houses so named by Lord Tavistock, the heir to Woburn Abbey. The other six are Castle Howard, Harewood House, Warwick Castle, Blenheim Palace, Longleat House and Beaulieu. All except Warwick Castle are still owned by the original families.

But be warned -- a good half of the houses you'll want to see are not included in either ticket. (Nor do they do much good at Westministe Abbey, where I longed for someone to sweep the moneychangers out of the temple. The altar and several chapels are fenced off. open only to those who pay admission -- separate fees for each attraction.) the fee at most houses runs between 1.10 pounds to 1.80.

We should have bought as well the combined ticket put out by the Historic Houses Association, available from them at 10 King Charles II Street in London. It costs about the same as the Open to View ticket. Lord Montague, past president of the association, promises that it includes all the other houses.

Another necessary expense at each stop are the booklets describing the castle or house. These are modestly priced, usually about $1.50 or $2. Books in general were the only good buys we found in England. Often the castle shops will offer other interesting books on the neighborhood or even, in one case, a biography questioning the legitimacy of the current owner. In one place I saw a map marking the location of castles and historic houses.

The American office of the British Tourist Authority (680 Fifth Avenue, New York, N.Y., 10019, also in Chicago and Los Angeles) is helpful -- more so than their London counterpart. They'll send you all sorts of brochures and lists of hotels, houses, or whatever you want to know. In Britain, the local offices in the countryside also seem to be well-manned or rather womaned with local people who are helpful -- for a small fee (under a $1) -- at finding you places to stay when everything seems full.

The biggest disillusionment we had was finding that all prices in England seemed scheduled for those who lived in stately homes rather than those visting them. The first blow was the car. We booked through Avis, a small Ford Escort. The bill, including collison insurance was 183.79 pounds, for 16 days and almost 1,000 miles.

Making matters worse was the Value Added Tax, which was doubled at the beginning of the tourist season. VAT is sort of a super salestax. It adds 15 percent to everything, from hotel meals to rooms to retail sales. The hotel keepers and restaurants seem to have taken the VAT increase as a chance to push up their prices by an equal amount. Both restaurants and hotels often add 10 to 25 percent for service -- even when there isn't any, and you have to carry your own bag up seven flights of steps. At least half the hotels we stayed in, including one of the most expensive, had no porter to tote the bag.

The leicester Arms, in the tiny town of Penshurst, was the cheapest hotel we found and our favorite. The price for a night was 20.40 pounds, when the pound was costing us $2.31-$2.39. The twin beds were comfortable. And it came with that most splendid of English hotel innovations. a kettle, a packet of teabags, instant coffee, and a few cookies. The hotel was several centuries old but the plumbing, unlike most places we stayed, was excellent: a real shower that neither boiled nor chilled, and a w.c. that actually flushed. Dinner that night was splendid, and seemed cheap at 10 pounds for two with beer for my husband and wine for me, after what we'd paid elsewhere.

The most expensive hotel rate we paid was 30 pounds, including breakfast, at the famous feathers Hotel in Ludlow. The half-timbered Elizabethean hotel was very quaint indeed, but the food was only so-so in the dining room.

We found that the price of the hotel had no relation at all to its facilities. Nor did it seem to matter what part of the country you were in. Brighton and London, which you might expect to find the most expensive, were no higher than some of the smallest towns.

In brighton,for instance, we stayed at the Regency Hotel, once a fancy "terrace house" as the British call their now houses, with a great view of the sea. We were only on the fourth floor, but the ceilings were so high there were two flights of steps for each floor. We felt as though we'd had our exercise for the week by the time we lugged up our bags.

But the housekeepers were very helpful and accommodating.

And the cool fresh breeze coming in from the sea was the most exotic pleasure of the trip. We paid 24 pounds. It would have been two pounds cheaper without the view. The sight of the old Edwardian pier was worth every pence. Next time we to England, we'll go to

Brighton first, and maybe even rent the Regency's "Edwardian suite" with the canopied bed (though the housekeeper worries about it falling on the boisterous guest).

Some of the hotel keepers have a new wrinkle in ways to tap the tourist. They'll book a couple in for a double room. Then after you check in, they'll say you can only have it for one night. The second night you'll have to take a twin-bed room. What they usually forget to tell you is that a twin-bed room costs two or three pounds more, plus the inconvenience of the move. This happened to us twice, once at the Wilbarham Hotel in London, where we had confirmed reservations at three pounds less. The hotel keeper finally gave us a better rate for half the nights, but it was still six or so pounds more than we'd expected to pay. Breakfast was not included, though our confirmation said it was. The other was. at the Cathedral Hotel in Salisbury, but there we declined the change and moved on.

The restaurant's ploy is to offer only a table d'hote menu, with a first course a bit heavier than Americans care to eat, such as seafood crepes, plus an entree and dessert. The price is suitably high, usually in the neighborhood of 5 pounds and up. You understand we're talking about the cheapest restaurants we could find. Often the stated price did not include service and VAT. Even in an Indian restaurant in London, they would not serve less than what they considered a full meal.

Worrying about our pounds -- in more ways than one -- we took to eating in pubs. But that's fraught with frustration. The pubs and restaurants are only open during certain hours when they're allowed to sell beer -- 12 to 2:30 pm., 6 to 11 p.m. So if you're driving, you must remember to stop in time. Sometimes pub food isn't bad. But far too many only offer grated cheese sandwiches. If you look pathetic enough they'll offer a jar of what they call pickles but we think of as chutney.

A friend who likes food asked why we didn't eat in the foreign ethnic restaurants. Well, they don't exist in the English countryside and they aren't too prevalent near where we stayed in London. The best meal we had in England was at Brighton at the Cypriana Steak House and Greek Restaurant, which specialized in "Meze," a series of tastes of wonderful Cyprus food. The price, even with two bottles of beer each, was only about 8 pounds.

We'd had a disillusioning lunch -- frozen fish fried right in view of the sea. We'd walked down Preston Street, looking for someplace to eat. Reading the menus, we soon discovered that even restaurants calling themselves "Scotch House, and such, were really Greek.

The best thing to do is find a grocery and buy bread, cheese and sliced ham, with a big bottle of cider to wash it down and perhaps a bit of mead for dessert. Just around the corner from our London hotel was a branch of Partridge's, a lifesaver on our last day when we were fed up with restaurants. pPartridge's even had a bottle of wine chilled to go with it. Next time, I'd be tempted to eat there every day.

The best restaurant in London, say some, is the Tate Galery, but you have to book ahead and you may have to share a table. Our meal was splendid, with generous servings of Elizabethean style recipes. Prices were not too bad -- 21 pounds for three people.

We certainly saw what we'd come to see -- 15 castles and English country houses, each more amazing than the one before. The turrets were as high, the moats as deep, the gardens as glorious, the gilt as glittering as we'd expected.My husband took 48 roles of film, and if your eyes are strong we'll be glad to show them to you. But be warned that many of the houses and castles will not allow you to photograph the interiors.

The guides in all the historic houses were as splendid as their houses. Sometime they were aging family retainers, sometimes they were aristocratic widows from the neighborhood. Most of them knew a great deal about their houses and were enthusiastic about sharing their information.

We found all the folk exceptionally kind about giving directions to lost Americans. One sportscar driver led the way around several blocks to get us where we were going.

All of this only applies to southern and middle England. We still have Ireland, Scotland, Wales and Cornwall to go. I'm reading up on their houses all ready. But I wish we could take the cook from Trudie Ball's Empress with us.