Is there any slim possibility that an accusation of torture “complicity” against MI5 could be lies?
I would hardly say that it was the responsibility of M15 to tell foreign agencies how to conduct themselves with suspected terrorists. There has never been an accusation that M15 actually committed torture. Lets be real about this however, if another country is holding a suspected terrorist and M15 have a vested interest in this man, than surely you would expect them to confer with the other agency and compare notes, so to speak. Is it our right to tell others how to conduct themselves, yes we can point out that torture is not acceptable. But this is real life with terrorists that kill anyone regardless and who themselves have no regard to life.
