"Can I help you?" asked the clerk at a London office of the Thomas Cook travel agency.

"Yes," I said, all pasted up in my I-am-about-to-make-a-joke smile, "I'd like to see some brochures on one-pound package tours."

She wrinkled her forehead in puzzlement, then saw the light and went digging in the shelves. In Britain, one-pound tour packages are the hot new item in the travel agents' stock. "One pound," of course, is not a pound in weight but in British currency - worth about $2 at present. For this you get a tour?

No not exactly. FOr this you get a play a new air-fare-game. It works like this: You go to a travel agent and you sign over accommodations you've supposedly arranged for yourself at a given city. They're then sold back to you for one pound. That's then a "tour package" - and makes you eligible for a greatly discounted air fare to the designated city.

So go to Britain and save a bundle compared to what you'd pay at home, right?

Not necessarily - and that, in effect, sums up a larger story: It's true what you've heard about air fare and tour bargains lying in the streets of London. However, that doesn't mean they're all big blessings for travelers on this side of the water, or that you're sure to get one. They have as many strings attached there as they do here, and they require careful, careful shopping.

This winter the "one-pound" tours, for instance, (1) are available only to a handful of cities, mostly Mediterranean sunspots; (2) go and return only no specified dates; (3) are available in limited numbers only, and (4) may or may not represent a saving compared to what you'd pay to go direct via a cheap fare from here.

But when they're good, they're very, very good?

Well, at the moment you conceivably could fly U.S.-London-Athens direct charter for $150 less than the scheduled airlines' advance purchase excursion - if you don't count the cost of a London stopover, if you can get all the flights conveniently coordinated, and if you can find a ticket.

Obviously, those are sizable "ifs." A number of British tour companies have opened offices in New York in order to remove one of the "ifs" by offering the advantage of nailed-down arrangements on at least some tours (although NOT the one-pounders and not discount air fares). However, to "buy British" profitably, it still helps to be lucky, flexible and smart.

Smart, in part, means that though we'll get to the good news in time, it will pay you to stay tuned and read the equivalent of the fine print starting here:

Just as they do in the United States, the vast majority of top-of-the-table, legally discounted british air fares come with "requirements." Alternatively or additionally, those that don't require buying a tour to get a low fare generally do require buying a round-trip ticket, paying for it one to six months ahead, agreeing to certain length-of-stay requirements or traveling during a certain period (by night, for instance, or on a weekday).

It takes time to shop anywhere, and it certainly takes moe time to ship where you don't know the ropes and have a lot that needs examination. For example, if "cheap flights" to Athens are advertised at round-trip fares of 48 pounds, 55 pounds, 75 pounds and 85 pounds (as they were recently, all in one edition of a London newspaper), you'll be some time finding out what such figures mean why they differ. Is the price for one flight in mid-February only? Is it a "come-on" rate to which you must add 18 or so extras? Will the ticket be delivered at the airport - maybe? Do you really fly the whole route, or does a bus trip get slipped in?

British tours haven't brought home entirely thrilled customers, any more than U.S. ones have. For guidance therefore, it's useful to note where things sometimes have fallen apart. According to a study by Britain's major consumer organization, complaints involving tours in all categories centered on last-minute changes of hotel, too-small hotel rooms, noisy areas, monotonous food, areas "not as led to believe by brochure," unhelpful or unseen tour escort, unanswered complaint letters and overcrowding. Note as well tha some British tour brochures do not specify hotels, merely a grade of hotel.

"Consolidation" of tours and charters is allowed under British rules, meaning that it's legal to transfer passengers to other flights on other dates and to change departure airports. Also, cheap long-distance charters, in particular, tend to be moved around and may not have a fixed departure hour even a few days before the takeoff date.

Surcharges for anything other than increased fuel costs are not supposed to be made less than one month before departure. However, even for tours with "guaranteed prices," it's advisable to ask what happens if and when exchange rates change.

Besides accredited travel agencies, London has a load of gray-market "bucket shops," some solid, some sharks. Basically, what the good ones sell are cut-price seats on otherwise-legitimate flights, the tickets having been slipped to them either by tour operators or by airlines trying to fill up a plane. This means you often can find a one-way cheap seat or a "tour" that allows you to skip the tour and buy only the air portion - expecially at the last minute. Sharks deal in flaky flights with hugger-mugger-sounding arrangements, and also sell full-fare seats at an additional markup to customers who have demonstrated ignorance of real rates and thereby revealed themselves as true turkeys, available for plucking.

The "supplement" charge is alive and well and living in London. Therefore, don't take a nice low tour price at face value. If you want a single room, a private bathroom, a balcony or a sea view, or want to travel during a major holiday period, it may be "extra." Check, too, to see what the story is on airport taxes and insurance. On some European flights, these can add as much as 15 percent to the state price. The British also charge a 2-percent fee to pay for a bonding scheme aimed at protecting passengers in the event of he tour operator's insolvency. (Even so, there's some question as to whether the plan would work if put to a major test such as the domino-like fall of many large and small British operators a few years ago.)

That's the not-so-good news. But if you're game and careful, buying British can pay? Yes, at times it can. And next week's article will tell when and how.