Whenever I get to London these days, barely a morning has passed before I find myself strangely compelled to take the Circle Line or the Number 14 bus to South Kensington, heading toward a peculiarly ordinary coffee shop on Princess Road. It's on the right, a few houses from the South Kensington tube station, next to a florist and a bookstore, and long familiar to my family and friends as the Rock Cake Cafe. (Familiar, no doubt, to the point of surfeit, as there's not a London visitor I don't send there, talking it up at great length. Generally they are disappointed.) The unofficial, familial name given to this cafe comes from a pastry that was served there some five remodelings, owners, short-order cooks, waiters, and waitresses ago, sometime in the early 1980s. It is in the shape of a lump or a patty and gives off a faint taste of vanilla but is otherwise conspicuously without any flavor whatsoever. But not even its "rock cake"unavailable for at