The National Trust; THE ART OF COLLECTING STATELY HOMES

ONCE every Briton had no chin, was given to ejaculating ''pip-pip'' at intervals, and was tewwibly, tewwibly rich and possessed a very stately home indeed. Or so many Americans were thought to believe.

Where this enormously inaccurate stereotype came from and where it has gone is hard to tell. But in one sense part of the stereotype has come true. Thousands of Britons (about 870,000), for about (STR)10 a year, now own a very small share of very many palatial homes (over 160 of them), thanks to the National Trust, a charity incorporated by act of Parliament.

Back in 1934, the Marquess of Lothian pointed out that with so much money disappearing in death duties, great houses were falling into decay and would eventually wither away. And so the Country House Scheme began.

It works like this: If I found I couldn't afford to maintain Marsh Acres, and the trust considered it interesting enough or beautiful enough, I could give it to them. I and my descendants could go on living there on most advantageous terms. I might make an endowment or make a present of the contents of the house , and live rent free or perhaps pay a nominal rent.

Of course I must allow public access and mustn't mess about with the character of the house. In fact the trust would prefer me to go on living there - it doesn't want the houses to turn into museums and failing all else will sometimes rent the house or part of it to suitable tenants.

The nation would acquire a house, I could go on living in my home, my heirs would pay no death duties, and the National Trust would maintain Marsh Acres ''to the highest standards in perpetuity.'' If Marsh Acres really existed I would be knocking on the trust's door forthwith.

Actually the houses are only one item the trust likes to collect. With 422, 000 acres including Snowdonia, bird sanctuaries, gardens, farms, lakes, villages , farms, and windmills, it is the third-largest landowner in the United Kingdom (only the state and the crown own more). Then there are such properties as mills (water and wind), villages, Hadrian's Wall, farms, the Stratford-on-Avon Canal, and England's oldest surviving landscaped garden (Claremont, Surrey). There is even a US branch, the Royal Oak Foundation, for Americans who care about Britain.

Of course you don't have to belong to the trust to visit any of its properties. For around (STR)1 (about $1.80) any visitor can see how the other half lives or used to live. In Washington village, Tyne and Wear, for instance, drop in on Washington Old Hall, where George Washington's family lived from 1183 to 1613 (considerably rebuilt about 1610), or take a look at Hanbury Hall near Droitwich, a red-brick Wren-style house built by Thomas Vernon around 1701 and hardly altered since. On its stairway and on the ceiling are far-from-comfortable-to-live-with dramatic murals painted by Sir James Thornhill - a sort of cartoon filled with huge struggling gods and goddesses. It has an exquisite collection of porcelain and, thanks to a donation from the US Merrill Trust, period furniture.

I might decide of course that I would rather not give Marsh Acres away. I could keep it and let visitors in on my own terms. That's what the Earls of Leicester have chosen to do with Holkham Hall. In fact, visitors have been welcome there ever since they were allowed to look around any Tuesday afternoon in the early 18th century.

Young Thomas Coke was only 10 when he inherited his estates at Holkham, in Norfolk. At 14 he went off on a grand tour of Europe, accompanied by his tutor and his valet. He gathered education, a friend (an architect called William Kent), and shiploads of treasure. In fact there was so much treasure that young Coke asked William Kent to help him build a house to hold it all. Holkham Hall is the result.

It's a palace, not a home - unless acres of marble and mosaics, priceless tapestries, ancient Greek statues, fine old masters by the yard, and libraries of manuscripts (including some Caxton, but the Leonardo was recently sold) give you that cozy, homey feeling.

It would be a pity though to skip over Gainsborough's portrait of Thomas William Coke (first earl of Leicester), one of Holkham's finest paintings. The first earl (of the second creation) was a great supporter of George Washington's , toasting him every night after dinner. But his loyalty went deeper than that. In the House of Commons in 1782 he proposed that, despite the king's opposition, Britain recognize the independence of the United States, and so formally end the war. The motion passed and the young earl (28) then presented the Commons's address to George III, shocking the court by appearing in country clothes. Gainsborough chose to paint him in those clothes.

So there you are -- for less than $2 at home among the splendors of Holkham Hall for an afternoon, enjoying the art, picking up footnotes to history.

Up and down the country, there are equally splendid houses you can take possession of. The National Trust (42 Queen Anne's Gate, London SW1) lists its ''Properties Open in 1981'' with times and prices of admission. The much fatter ''Historic Houses, Castles, and Gardens (World Timetable Center, Church Street, Dunstable, Bedford) includes almost every house open to the public -- trust or nontrust. Alas, ''Marsh Acres'' is not included in either of them.