text: Video Said to Show British Hostage Killed BAGHDAD, Iraq - British hostage Kenneth Bigley, who pleaded tearfully last month for Prime Minister Tony Blair to meet his kidnappers' demands, was beheaded by his captors, a witness who saw the videotaped killing said Friday.    The witness, who asked not to be identified by name, said the tape - sent to Abu Dhabi TV - showed six hooded, armed men standing behind the kneeling Bigley, whom the witness recognized from two previous tapes released by the kidnappers in which he pleaded for his life...