BRITONS HAVE A DOGGONE GOOD TIME WATCHING SHEEP-DOG TRIALS

People roll up in their Range Rovers and green Wellington boots and stand around on grassy banks in the English Lake District or the Welsh hills to watch a few rounds of some local sheep-dog trial. But the thing that has turned sheepdog trials into an increasingly popular spectator sport in Britain is a TV program called ''One Man and His Dog.''

Thanks to it, vast numbers of Britons are now familiar with the intricate hazards of sheep-dog trials, from the outrun, the gather, the drive, and the cross-drive, to the shedding ring and the pen.

Now airing its 18th annual eight-part series (back on the shores of Buttermere, in the Lake District, where it first began), this Sunday-evening, 45-minute program on BBC-2 attracts some 7 million viewers.

This popular program could be one of the corporation's best-kept secrets. Scottish shepherd Jock Welsh has judged trials all over the United States. Growing US interest would suggest a potential American audience.

Oddly, however, the show is not sold outside Britain. Producer Joy Corbett believes the BBC has a tendency ``back-room it.'' Welsh himself appeared in the first round, the Scottish heat, of the current series. He ran his dog Craig, and placed second by a narrow margin. A philosophical loser, he stayed on to watch the subsequent rounds. The overall winner will be known Nov. 13. Still a largely male sport, this year's winner will certainly be a man: The only woman to win so far was Katy Cropper, in 1990.

The truth is, though, that it is not the human who actually wins. It is the dog. With a little help from his friends.