Britain's first new cheese in 300 years -- cultured for the cultured only

In the United States, class tends to be the size of the home, the clout and the type of the job, the model of the car, the ivy on the university, the antiquity of the family tree.

In Britain - well, that's easy. Class is the accent, of course, the family, the stately home, the clothes, the cheese.

Hold on - the what? The cheese?

Indeed. Telling a man by the cheese he eats has emerged as a definite class issue here following the launch of a new British variety - not just an ordinary cheese, mind you, but the first new type of cheese to be sold commercially here for 300 years. Since Stilton first appeared, to be exact.

Class now has a rind on it. The new cheese is called, with an eye to sales, Lymeswold.

Its publicizers, the British Milk Marketing Board, wax lyrical: ''full-fat, soft, blue . . . encased in a white velvety rind . . . the delicate buttery flavor enlivened by a subtle, yet distinctive tang. . . .''

As any viewer of ''Upstairs, Downstairs'' will tell you, class has long been what you eat as well as who you are. Some people eat pheasant under glass. Others order burgers under bun.

The point about the new cheese is that it is soft. Surveys show that the British middle and upper classes eat soft cheese, while the lower orders, by and large, do not.

Until now, the British have made mainly hard cheeses - Cheddar, Cheshire, Leicester, Double Gloucester, Lancashire, Wensleydale, and the rest. Only 3.6 percent of the 242,000 tons of cheese made here last year could be called in any way soft.

Surprisingly, the British themselves eat less cheese per head than most other Western countries. No one rushed to bring out new varieties.

Like others, the cheese market is upwardly mobile. The cheese boards, the patio snacks, and the dinner tables of those with the substance or even the hope of style have been left wide open for the Europeans to exploit. Leading the assault have been the French, widely regarded as a sinister force in British cheese circles, with their subversive Bries and Camemberts.

Now Britain is fighting back. The battle for the upper- and middle-class palate is joined.

The Lymeswold launch came complete with television advertisements sounding a World War II theme. They showed Englishmen turning road signs around to fool French invaders trying to steal Lymeswold secrets.

A beaming Peter Walker, minister of agriculture, predicted instant and constant success. Officials report gratifyingly large initial sales.

Not a word about the class implications, of course - simply not done, old boy. ''No one set out to make a cheese for rich people,'' a board spokesman said.

The price, however, is comparable to Brie, and considerably higher than Cheddar. The board has spent $:5 million (about $8.5 million) to develop the cheese and $:3 million more on a creamery in Cannington (not Lymeswold) in Somerset. Tips on how to buy and serve dwell heavily on ideas far removed from the average blue-collar home.

One suggestion is to serve Lymeswold on half a pear laid on a bed of lettuce, and garnished with cucumber slices and green olives. Another mixes Lymeswold with yogurt, salt, and black pepper, folds it into gelatine with lemon rind and egg white, and lays it onto slices of smoked salmon.

In fact, the British market research bureau has found that 67 percent of so-called AB (professional and executive) consumers claimed to have eaten blue-veined cheese, while 66 percent had tried soft cheese. Down in the DE (unskilled worker) grades, 25 percent had tried blue, and 36 percent soft.

But it's not just British customers that the Milk Board has in mind. The board wants to add British insult to French injury and export Lymeswold to France as well. Test trials are said to have been held in France already, under a different name.

Americans may also be eating Lymeswold before long. The British are chagrined to discover that Danish Blue outsells all British cheeses combined in the United States.

''Don't cheat on the cheese,'' exhorts an advertising campaign here to boost sales of all cheese. ''Eat Lymeswold,'' pleads Mr. Walker.

It is pointed out here that it will be hard cheese for Britain if the British people do not - and hard cheese for France if they do.