The US military kangaroo courts have stated many they have released to no longer be a threat, yet take a look at how many proved themselves to be (most are now dead thankfully).

As to the argument of him being in UK Custody I do think that that has come up yet as their first take on thinks was that your MOU has long since lapsed.

UK may request all it likes, if the US says no then that is that, perhaps the Pakistani's will have more joy when Reprieve go for the release of their Yunus Rahmatullah the one they transfered to bagram in 2003, after all if the details relating to his arrest and transfer are accurate he can hardly be -

"Yunus Rahmatullah, a Pakistani national, was seized by British soldiers in Iraq in 2004 and has been kept at Bagram airbase in Afghanistan after admitting he wanted to take part in "jihad"."

i.e. the Yunus Rhmatullah who Reprieve in the UK want returned.