Do-it-yourself decors flourish in British design showrooms

''There's still such an anti-decorator bias in England,'' says Billy McCarty, an interior designer who lives and works on both sides of the Atlantic. ''The British housewife still wants to do it herself.''

Fortunately, the British system of marketing home furnishings allows - even encourages - this do-it-yourself approach. Unlike American showrooms, where discreet signs on the window often read ''To the Trade Only,'' or ''Wholesale Only,'' shops here make no attempt to separate customers into two camps: those who work with an interior designer and those who do not. From the famous chintzes at Colefax & Fowler on Ebury Street to the hand-print wallpapers at Osborne & Little on Kings Road here, even the best lines of fabrics, wallcoverings, furnishings, and accessories are readily available to any retail customer.

Interior designer David Hicks explains. ''You have that (closed-door) system, and nobody can go into a wholesaler,'' he says, chatting with a group of American visitors in his elegant London showroom. ''Here, any little character off the street who wants to come in and buy one small accessory is as welcome as the lady in a mink coat who can spend millions.''

To illustrate this accessibility, he tells the story of a woman who ''came in before Christmas and said, 'I want the right-hand window.' She bought the lot. She was in a hurry.''

Most customers, obviously, are neither that rich nor that rushed. But the point remains: An open-door policy can benefit both buyer and seller. Among other advantages, making products easily available to retail customers seems to create a friendlier, more welcome feeling in design studios. And as Tricia Guild of Designers Guild in London points out, it also helps to ''demystify professional interior decoration.''

''The more people know what quality is, the higher the quality that will be available,'' says Mr. McCarty.

''I think the American way is ridiculous,'' he adds. ''It's encouraging a lot of '10 percenters' (people who register as decorators to qualify for a 10 percent discount) to call themselves decorators. They clutter up the showrooms as much as any retail client. I don't see it hurting the professional designers (to open showrooms to retail customers). Professional designers will use the same materials in a different way.''

''Taste in the US has grown so much in the past 15 years,'' Mr. McCarty continues. ''I think it's criminal that the shelter (home and garden) magazines show people so many beautiful things, and then people don't have access to what they see. The shelter press has done such a good job that it's time to change the approach.''

Many American shoppers would agree.