Presumably part of the deal was that the police would go through the motions of pretending there was some reason to think they might be guilty of something.

I imagine this was a return for Blair helping Bush in the pre-election days, when he pushed the Black Watch in, to help get things ready for the attack on Falluja. The idea would be that with an election coming up, these releases might take some of the heat off. However the hundreds still in Guantanamo Bay still include a number of British residents with British families, in respect of whom the British Government has washed its hands so far as any help is concerned.