THE BRITISH Embassy, despite many challenges in its 50 years, has remained the most imposing foreign building in Washington. Both chancery and residence, it looms Forever England above Massachusetts Avenue. In its presence, other embassies look shy and unsure of themselves.

With its imperial presence, suitable for ermine robes and crown jewels and processionals of powdered wigs and beefeater hats, you expect change to come slowly to the British Embassy.

But this week, at an invitational gala (international events permitting), Sir Nicholas and Lady Henderson, will pull the curtains and reveal an almost totally redecorated residence, a decorators' show house of British fabric, wallpaper, furnishings and design. Most of the materials used in the redecorating are available in the United States, and most of them are in the moderate price range.

Eight British interior designers are responsible for individual rooms. Lady Henderson herself and Suzanne Gilligan of the embassy's staff have designed seven rooms as well as coordinated the enormous job.

The other morning, Lady Henderson and Gilligan sat in the cheerful morning room, decorated with "English Country Garden," a fabric by Bailey & Griffin, and talked about the transformation. At that stage, the ballroom and the drawing room were still at the ladder-and-paint stage, with all the work not expected to be completed until Thursday's event.

"I believe that our embassies should mirror the taste and materials of Great Britain," said Lady Henderson, wearing a dress made of the same fabric as the morning room's "English Country Chintz" curtains. (She has dresses in Liberty and Laura Ashley fabric as well.) "When Sir Edwin Lutyens designed this house, he thought of it as a grand English country house. We've tried to restore it to that idea."

In England, a country house is not a cottage in the country. It is a country seat, what Italians callSee BRITISH, Page 2, Col. 1 Ruffles & Flourishes The great columns that march down the hall of the British Embassy were restored to their original appearance after years under layers of paint. Photo by Douglas Chevalier--The Washington Post. BRITISH, From Page 1 a villa, and the French, a cha teau. It is the principal residence of the ruling lord of the manor. Such houses evolved from defensive castles after the country was more or less united under the monarchy. The greatest country house architect of the 20th century was Sir Edwin Landseer Lutyens, who even built an overblown English country house as the Viceroy's Palace in New Delhi, India. The British Embassy here is a more modest version of the New Delhi extravaganza.

All such houses, whether English baroque (a particularly heavy extravaganza, deep in velvets and gilded ceiling) are set in expansive gardens which often seem to be treated with more attention than the house. The gardens are frequently precise and proper with no weed in sight while the house crumbles in the corners.

Lady Henderson's concept of an English country house is romantic. She has brought the garden indoors, filling the curtains, upholsteries, bedspreads and tables with bouquets of English flower designs. Pattern is piled upon pattern, to make the whole house seem to have moved further back than 1930 when it was finished. It's back to the Victorian taste of More Is More.

The feeling is abetted by the Hendersons' personal collection of Victorian sheet music covers, photographs, plates, ashtrays and objets d'art. The colors too are Victorian, no true pure, primary colors, but rather all with a firm admixture of gray. So the reds are ports, the greens are the color of dried herbs. They rather look like colors seen through the mist of a British November. Ruching, flounces, billows and pillows are everywhere. Every table in the bedrooms wears a chaste long skirt with a ruffle on it. Every chair has a wardrobe of pillows. Curtains are often lined in the room's secondary pattern. Curtains divide the long upstairs hall into more intimate sections and festoon the baths.

Lady Henderson's project, she points out, differs from a traditional English house in that the redecorating is largely happening at one time, "whereas normally, one would have new curtains perhaps, or new upholstery, but hardly the entire house all new."

The project began, Lady Henderson said, because the house "needed to be done up." Lady Henderson had served as honorary chairman of the Washington Symphony Decorator's Show House, wherein each room was decorated by a Washington designer. "So, I thought, why not try something similar to make the residence a show case of British materials, so many of them now imported into the United States."

Lady Henderson went to London, and before long, she had most of the principal designers signed up to do a room apiece without charge. Then the British manufacturers agreed to give the fabric, wallpaper and occasionally pieces of furniture. "The Foreign Office would never have paid for all of that," she said.

The major improvement is restoration, not redecoration. The great columns in the ballroom and marching down the great hall of the house were originally intended to look like sienna stone. The columns are metal, finished in a technique called scagliola, an imitation marble composed of plaster of Paris, isinglass, marble chips and paint. The method goes back to ancient times but was revived in the 17th and 18th centuries. In recent years the columns had been painted over. Lady Henderson was told that they'd probably been painted for good reason, that undoubtedly the decoration was damaged.

"But I persisted," Lady Henderson recalls. "We had one stripped of paint, which came off quite easily, and it was perfect. Well, then they said probably the next would not be right. But we kept on, and as you see, they are all fine."

The columns and the two mantelpieces in the ballroom are all in an ochre/gray faux finish. The walls are white. The pilasters, a darker, stone shade, match the ornamental goddesses' heads of the frieze.

The ballroom has always had panels of glass alternating with pilasters and plain panels. In researching the original decoration, Lady Henderson consulted photographs and memories of people who had seen the house years before. She found that the glass panels had originally been a smoky black glass. She was wondering how to duplicate the effect when one of the embassy's painters said the original glass sections were up in the attic. "We thought surely many would be broken, but we only had to duplicate three or four."

The designer of the ballroom's redecoration is John Stefanidis, originally from Alexandria, Egypt. He now has a vacation home in Lady Henderson's native Greece. He is one of Britain's major designers, though he often works in the United States.

Not everything Stefanidis suggested is being done--he wanted candles in the three spectacular Austrian crystal chandeliers. "But we could never do it, for safety reasons," said Lady Henderson.

Stefanidis, in town to plan the room, admitted that even if you are following an English country house theme, you have to make allowances for "the American light. In England, most of the time, there is no light. But in Washington, everything is so bright. The light is intense here and the humidity is a consideration."

The design of the ballroom was particularly difficult because of its multiple uses, everything from a sit-down dinner for 300 to parties for 90 children or a reception for 600 members of the staff and their families. "And then," said Lady Henderson, "when we have 32 for dinner, that's too many to sit comfortably in the drawing room. It will only hold 12 or 14. So we have to come into the ballroom."

So Stefanidis has set up a plan for five conversation groups, each treated individually in fabrics by Stefanidis, Colefax & Fowler, Bernard Thorpe and Marvic Textiles in a rainbow of colors--red, cream, yellow, orange, pink and brown--all against a white background. One fabric, Ladakh chintz, was designed by Stefanidis just for this project.

In front of one fireplace, the two matching sofas are in Stefanidis' gold Varese, the other sofa is Colefax & Fowler cream geranium moire with a large goatskin rug made for the room between the two. A group of French classic side chairs with a pink and green and beige stripe is by Marvic Textiles. The curtains are coral and white pinstripe lined with a lemon and white pinstripe "Regatta," a silk from Tamesa.

The 40-foot-long Tabriz rug came originally from the Great Exhibition at the Crystal Palace in London (1851). It was used earlier in the former embassy on Connecticut Avenue.

The downstairs library was designed by David Hicks, possibly the British designer best known in the United States. The colors are sort of Pompeii red with blue. The roman shades on the arched windows are in Hicks' parrot fabric. The color sets off the room's elaborate wood caving in the Grinling Gibbons manner. The carpet has a small geometric, also designed by Hicks.

The drawing room by David Mellanaric is being painted from the cornice to the floor in blue, according to Gilligan. All the fabrics are Colefax & Fowler. The curtains, valances and window seats are blue "Henrietta," trimmed in a brown binding. The sofas are Diamond Fawn. The easy chairs are covered in blue-brown Aconite.

Upstairs, the most unusual room is the sitting room that adjoins the principal bedroom, first redecorated in Laura Ashley fabric for a visit by Prime Minister Margaret Thatcher.

Lady Henderson and Gilligan have used Laura Ashley flowered burgundy ground chintz for the bedroom and striped wallpaper for the walls in their redecoration of the sitting room. Easy chairs are slipcovered in burgundy with a small triangle of dots. The flowered chintz curtains are lined in the striped fabric on the wall. John Lightfoot, the butler, keeps the Toby vase and the small copper pots filled with flowers. The other day a basket was planted with daisies.

In all the bedrooms are round tables covered in fabrics matching the rooms. In this one, Victorian photographs are arranged in a frame.

The principal feature of the room is the floor, painted in red and white marbleized squares, a pattern suggested to Lady Henderson by the long corridor on the main floor. Malcolm Robeson, an Englishman working in Washington, did the marbleizing and laid out the floor. "We suggested he measure the squares in marble in the hall and he came back and said 'which square?' Lutyens designed the hall with smaller squares toward the end to make them seem longer."

Robeson also "dragged" or as Americans say, striated, the walls in the bedroom across the hall, designed by Jean Monro and executed by John Byrom, another English designer who has a studio here. The headboards and chaise longue have ruching in a celadon polyanthrus chintz with pinks, green, violet and burgundy flowers. The curtains match. The bedspreads are Featherberry chintz. The carpet by Mrs. Monro Ltd. has a border of green and beige with red flowers.

Another sitting room, designed by Lady Henderson and Gilligan, uses Liberty of London fabric "Strawberry Thief" (brown birds, blue leaves and strawberries) and a Pre-Raphaelite William Morris print. The other pattern in the room is called Willow. Two miniature rooms, part of Lady Henderson's own collection, are set into the bookcases.

A charming kitchenette, which forms part of this suite, was the work of Dennis McGee of Conran's. Conran's supplied the pine kitchen cabinets, china, accessories, baskets and pots. The wallpaper and fabric are Mignon/Vymura by ICI Americas Inc.

As for what wasn't done--Lady Henderson, showing remarkable restraint, did not redo the master bedroom suite. "We've spent two years in bedrooms that I hate," she said. "I wouldn't wish it on the next ambassador and his wife. We expect to leave in the summer, so I thought our successors should have the right to suit themselves.

"Embassy Information

The British Embassy residence is not open to the public. But for more information about the fabrics and their distributors, call the British Embassy Trade Promotion Section at 462-1340.

Designers, executors and suppliers included: Anthony Browne Inc., fabric restorers and interior designers of Georgetown and London; John Byrom, interior designer of Washington and London; Bernard Neville, British textile designer; Malcolm Robson, master grainer of Washington and London; Tricia Guild, of Designers Guild, fabrics and wallpaper designs; Charles Hammond Ltd., British interior design company as well as country chintz designer and furniture wholesaler; Laura and Bernard Ashley fabric and wallpaper designs with shops in Washington and in 99 other places; Mrs. Monro Ltd. and Jean Monro Ltd., designer and decorator and chintz supplier; David Mlinaric, Chelsea interior designer who often works for the British National Trust; John Stefanidis, London designer; David Hicks, decorator designer of cars, yachts, houses, and materials; Robechar Decorators of Bethesda, who made the curtains and upholstery.