Look, that could be true for all we know, but there is no way in hell they'd know this in such short time. In fact, if they did know this, they'd end up looking very bad, as it would mean that they had reasonable intelligence that he was planning something yet did nothing.
The Brits are still processing the crime scene, and while I'm sure they have some "likely candidates," there's no way for certain that they know just who it was.
So to say it was a recent Gitmo inmate is just speculation at this point.
I was passing on information that was reported. Of course, I personally, don't know who did what and neither do you!