London — Nina Campbell, a well-known London decorator, is enthusiastic right now about a striped fabric that is a look-alike for mattress ticking. It comes in several soft colors and costs about $10 a yard. She likes it best on walls. She uses it in the shops she decorates as well as in homes, but actually, she says, it looks good anywhere. The idea is to use the material ''with a little splash'' and warm it up with braid trim.
A favorite of hers is gray-and-white striped ticking, highlighted with butcher blue braid and accessories. In her own Walton Street shop and design studio, she trims the gray ticking stripe on the walls with bright red braid.
''I find nothing at all wrong with using inexpensive materials,'' she says, sitting at her office desk. ''The secret of success is in using them expertly and in seeing that draperies, curtains, or slipcovers are well made, and don't look as if they were run up by the village dressmaker. Good fit and good tailoring are everything. I have seen $90-a-yard fabrics look like rags at the windows because they were not properly made, which is not only a waste but a disaster.''
In her Paris studio, the designer says, she used sateen curtain lining materials costing less than $3 a yard to make festooned blinds to cover the huge expanse of windows. She bound the edges with dark red. ''They cost so little, but they look so chic,'' she notes.
Her other fabric favorites right now are the French prints taken from 18 th-century patterns. ''They are more detailed and have more character than the 'wishy-washy' English fabrics that are available today. Those mono-print, one-color things with too little detail bore me.''
She happens to be an admirer of Laura Ashley but wishes people wouldn't plaster her tiny little prints all over great big rooms. ''They go to nothing at that scale. I always warn people against choosing a fabric from a small sample. It just doesn't work.''
Asked about some English and American differences in decorating (she works on both sides of the Atlantic), she comments that the English don't decorate nearly as often as Americans seem to. ''We have neither the inclination nor the money to do so. Neither do we pay too much attention to what is considered fashion at any given time.
''We pay little attention to a decree that everything will be peach color this year, or pink and green next year,'' she continues. ''We couldn't follow such dictates even if we wanted to, because our light is so different in England. We couldn't live here with all your Florida and California colors. They are too strong for us.''
She explains that days are often gray in England and it is frequently rainy. ''This fact of weather influences enormously the way I decorate for people. I feel they don't want to sit in washed-out rooms. They need a little bit of punch , and pattern and texture. Certainly our rooms need considerable warming up with materials and design.''
People in England, she is sure, are far more aware of the decor of their homes now than they were 10 years ago. ''I get enormous response from the magazine articles that feature my work. People have learned that they can adapt the decorator rooms they see in publications and translate the ideas at their own level.''
Nina Campbell is known in England for a kind of comfortable English country-house look that she also applies to city flats. ''I hate doing a house from scratch, down to the last paper clip,'' she admits. ''I love working with people's possessions that they already have. Nothing gives me more pleasure than recovering someone's favorite old comfy chair. What people already own is so frightfully important.''
Mrs. Campbell also doesn't at all mind working piecemeal: ''I say to my clients, 'If you can't afford to do everything, let's do one or two things properly.' Everything is so expensive today that I feel it is better to concentrate on one room at a time, and wait to do a second one later.''
She lives in a London mansion flat with her husband and three children. She maintains that children come before decoration. ''I do try to keep one room civilized and acceptably orderly. Otherwise, I don't mind all the litter that children make. My bedroom is full of all those little things that children make at school and bring home to their parents.''
A London colleague who admires her says of Mrs. Campbell, ''There isn't an ounce of chichi or push about her. But she does have that uncanny ability to see inside the minds of her clients and somehow to mold a loving home around them.''