Terrorists plan to kidnap Queen Elizabeth--in a sometimes bloody but mostly tame and predictable international thriller. Moslem prof Ibn Abdullah is the revolutionary founder of Bloody Christmas, a gang of terrorists named for the Sadat-Begin Christmas meeting and comprised of P.L.O. fanatics, Baader-Meinhoff leftovers, I.R.A. incendiaries, plus a member of the Japanese Red Army. Their plan: the two German hotheads, the two Irish firebreathers, and the really deadly Japanese killer will invade Buckingham Palace, secure Her Majesty and her lady-in-waiting, announce their demands, leave the palace by helicopter for a waiting Concorde at Heathrow, fly to Yemen, and then release the Queen. The ransom: the release of nearly 150 imprisoned terrorists of varied nationalities (thus providing the personnel for an Arab-funded, Yemen-based terrorist super-brigade). As it happens, however, their holding action within the palace is successful, but then a counteraction is mounted by a mixed bag of police, military, Israelis, and a Japanese. Unfortunately, Queen Liz herself speaks not a word throughout, hardly appears, and is never mentioned by name. And that's too bad--because some good-humored use of the royal personality might have given some lift to these decently written but thoroughly routine terroristics.