JUBILEE by Robert McCrum (Knopf: $22; 225 pp.) He's offstage, the Beaver is, for much of the story. Just as well; he'd burst the book. The Beaver--retired Admiral Sir Ronald Lefevre--is one of the last Great White Hopes, an unapologetic British imperialist. "Brave men with fire in their bellies," he blusters, "not a master race but a masterful one." Dying he is, but still stirring the pot, still raising Hobbes, with Northern Ireland as his valedictory. Having grudgingly unbuckled their swashes with the retreat of colonialism, the Beaver and his ruling-class cronies have turned to a more modern but no less deadly weapon--disinformation. And while their Belfast operation has "failed" in a conventional sense, they've achieved their ends: Just look at today's British government. Alas, Robert McCrum, for reasons of his own, has focused his novel on an etiolated imitation of the Beaver: his half-Yank son. Seymour, former speech writer for Jimmy Carter (and didn't Dad love that!), is keeping a promise to one Jam. Craig Marshall. A stolid Scotsman, Marshall has been jailed on a trumped-up charge of murder, trumped up because he too was involved in the Belfast ops and knows too much. The trail points to the Beaver--who really doesn't give a damn--and will lead to a Hobbesian's choice for his son: the truth vs. loyalty to the Old Man. In the mix is Ruth, svelte and sexy Australian "journo" who has a choice of her own: the boy or the Beaver. To McCrum's credit, we are kept in the dark, politically and romantically, until the end. To his discredit: too much Seymour and Marshall, not enough Beaver and Ruth. Nicely done, though, with some memorable lines. A favorite: "Prostate, c'est moi. "