Britain's PMs: the good, the bad, and the so-so; Prime Ministers, by George Malcolm Thomson. New York: William Morrow & Co. $15 .95.

The history of England's prime ministers, from 1721 to the 1980s, is the story of a remarkable assortment of 48 men and one woman, who have steered that nation through its magnificent moments as well as through times of decline.

Beginning with Sir Robert Walpole and ending with Margaret Thatcher, this book traces the careers of the people who have led 70 administrations. Eighteen were educated at Eton. Oxford shaped 24. Fifteen had a common ancestor.

The author successfully surveys this ever-evolving and increasingly powerful role by briefly sketching each officeholder's character and achievements -- virtues, faults, and misfortunes, as well as political successes and debacles.

Thomson has tried hard to ensure that the book is not dry reading. Intriguing facts are mixed with uncanny but documented anecdotes, spiced with quotes of consequence, and graced by the author's witty and charming writing. The text is supplemented by an ample selection of photographs.

Thomson characterizes his subjects as mostly, "dull men and a sprinkling of geniuses."

"As [is] to be expected," he writes, "the Prime Ministers of Britain have been a mixed bag, not conspicuously brilliant or imaginative, although displaying among them the occasional streak of something that can only be called genius . . . ." $ TFor readers who want a fast-moving survey with wit and style, this is a good choice.