Reply #4: They were never charged with anything.

[Bridget Burke]

(From the article)

Peter Clarke, head of the Anti Terrorist Branch, said police had discussed the case with representatives of Britain's Muslim community and recognized there were strong feelings about it.

"But the fact is that we have an absolute duty on behalf of all communities to investigate the circumstances leading to the men's detention," Clarke said.

So, the British are "arresting" them because they were detained.

At least these men had the British government behind them, to convince the US that they should either be tried or be released. Most of the detainees have no such backing. They haven't been charged, either, but the US can apparently hold them forever.