Open Tuesday through Saturday for lunch 11:30 a.m. to 2:30 p.m., for cocktails 4:30 to 6:30 p.m., for dinner 6:30 to 9:30 p.m.; Sunday lunch 11 a.m. to 2 p.m., dinner 5 to 9 p.m. AE, MC, V. Reservations. Prices: At lunch appetizers $1.50, main dishes $4.50 to $6.50, desserts $1.75 to $2.75; at dinner appetizers $2 to $4, main dishes $9.50 to $15.50, desserts $1.75 to $2.75.

This time of year it hardly matters what the restaurant is, as long as it's an hour outside of Washington: a drive in the country rewarded by Sunday brunch, a Saturday night out that starts midafternoon with the convertible top down, a few hours of vacation that don't require tangling with the Bay Bridge traffic.

But not any old shoppping-center continental restaurant will do. A drive in the country demands a dining environment more bucolic than Connecticut Avenue--preferably a trip into another age as well.

Windsor House fills all the qualifications. Middleburg's Washington Street is a 19th-century set with a centerpiece of an imposing double-chimney white-brick mansion that recently was a French restaurant but nowadays is the Windsor House, an English- style restaurant and pub. Only parking meters intrude on the architectural purity of this small main street. And the restaurant carries the Federalist environment inward. Three rooms and a pub, with not more than 70 seats in all, are paneled in dark wood or painted in muted tones, focused on fireplaces and lit by a few sconces and chandeliers. Wood tables, bare except for flowered mats, are decorated with modern but well-suited glass candleholders and glass bowls with a single flower. The carpet is plaid, the chairs are a variety of high-back, low-back and wing, and the waitresses wear tartans. Very British, very cozy, very far from downtown bustle. And on a balmy evening the lacy white metal outdoor tables and chairs can transport you even farther.

Ah, but what's to eat? Roast beef and Yorkshire pudding, of course, and grilled quail and mixed grill, on the British side of the menu. And there are a few other-continent additions, changing daily: perhaps softshell crabs or veal or ham or smoked pork chops. A small menu, beef-heavy but with stuffed trout balancing it, and always a few changes, as well as a few twists to raise it above the routine.

To start, the drink specialties include concoctions rarely seen outside a British pub: draft lager with ginger beer or with tomato juice; five beers on tap, from Harp Lager to John Courage. There is also cider and Pimm's Cup, and a surprising Watermelon Spritzer, which apparently has nothing to do with watermelon except for its color, and tastes like a first- rate Planter's Punch. Wines are less enticing here; the list is broad, but expensive for such second-string offerings.

The cooking will not knock your socks off, but it is competent and careful, leaving very good ingredients to speak for themselves. Seasoning tends to be faint; this, is, after all, cooking in the British tradition. Even salt is used with restraint in this kitchen. Some will find the food delicate, others bland. An asparagus soup, for instance, is homemade and tastes soothing, though its asparagus flavor is understated and it could use strengthening. A,e of veal and pork with diced Canadian bacon and hazelnuts is similarly mild (as well as large enough for two), a gentle sort of,e, its main character being textural--the crunch of the nuts against the soft and light meats. Shrimp cocktail here is the most inventive appetizer: the shrimp are marinated with a light touch of herbs and vinegar, then prettily centered on lemon slices and greens. It easily surpasses the standard tasteless shrimp cocktail found at most restaurants.

It was beef that tempted us on the menu. Standing rib roast was brought commendably rare, sliced thinner than a steakhouse cut but thicker than the paper-thin English style, and like the other dishes it was generous. With it came Yorkshire pudding that was as good as you might make it at home, which is to say far better than I have found at almost any restaurant. Light, eggy, crusty and tender, it was bested only by the muffins that are served as the house bread. Few things masquerading as muffins equal these soft yet springy golden ones, their texture somewhere between biscuits and scones.

The star of the menu, from what I tasted and saw, was the mixed grill. No measly mishmash, this. A sizable, thick and tender slice of beef filet, an acceptably thick and well- trimmed rib lamb chop, several rashers of crisp bacon, a plump sausage in the English style and a kidney if you request it, all charcoal grilled with enough attention that the beef and lamb were left as rare as requested, and decorated with a beautifully fluted mushroom and an uncommonly good stuffed tomato. The mixed grill was flawed by a faintly singed taste, probably from grill flare-ups or from not keeping the grids clean. But it was a fine presentation, even better if you ask for a sauceboat of the mint-flavored sauce paloise that is served with the rack of lamb.

Lunch is even more British, and perhaps more interesting a menu, in addition to the main dishes' being less than half the price than at dinner. There are burgers and a chef's salad, an egg dish and the,e, but those are just a start. Windsor House does an exceptionally good fish and chips, the fish delicate and steamy, not dried out in the frying, and the batter just a puff of crispness. The chips--the same "game fries" as at dinner--are freshly made but have little taste. The English Pub Lunch consists of two small meat pies, one of beef and one of veal, in an outstanding flaky pastry, accompanied by a bowl of salad with diced cheddar and an excellent dressing. The meat pies, like most of the food, are barely seasoned, and the pallid-tasting cup of sauce that accompanies them adds little.

Don't look for dazzle, but for good simple food at Windsor House. Dessert is less British: cheesecake, carrot cake, chocolate mousse, as well as the more traditional trifle and cheeses with fruit and crackers. The staff loves the trifle, and it is indeed good, with a light custard on the scotch-and-sherry- soaked cake and lots of whipped cream. They also love the strawberry shortcake, but then they couldn't know that ours had biscuits--homemade, to be sure--that were too hard to break with spoon or teeth. We loved the chocolate mousse, lightly minted and served in a tiny glass cup.

Windsor House is not so wonderful that I'd send you driving from Washington just for it. But it is good enough to enhance an otherwise-oriented trip into the country.

In case you like Middleburg enough--or hate night driving enough--to want to stay, Welbourne is an adventure, as far as hotels go. About six miles from Middleburg, it is a private home of magnificent proportions. It has nine rooms and three cottages to rent, from $25 to $50 per person a night, a fee that includes a glorious Southern breakfast right down to the grits and fried tomatoes, beautifully and graciously served from the family silver. Rooms are 18th-century beauties with four- poster beds, fireplaces and here and there such treasures as Chinese porcelain lamps and gilded miniature clocks, perhaps a Wedgwood ashtray in the bathroom, a chaise with a wool throw in the bedroom. It sounds marvelous and is, with two exceptions. Although the rolling hillsides and giant trees imply a quiet getaway, barking dogs replaced the noise of city buses for us. Even more disappointing, though, was being asked Sunday morning at 10:30 to vacate our room by 11. So much for a lazy Sunday morning. Then again, such a setting would be hard to vacate any time. Welbourne can be reached by telephone. 703/687-33201, or by mail Middleburg, Va. 22117.