Just because the British government - or any government - says something is the case that doesn't mean it is the case.

Just because the British government - or any government - or any NGO - or any individual - says something is the case that doesn't mean it is the case.

People do lie.
If a minister makes a statement that proves to be wrong, there are consequences.
If he makes a statement that proves to be a deception, there are more serious consequences for him.
They avoid statements that make them hostage to fortune.
This statement was unequivocal.
Is there a motive for lying?

If a captured terrorist lies, there are no bad consequences for him whatever happens, and he certainly has a motive.
He might, with the help of some gullible do-gooders, get to kill some more unsuspecting and undeserving people.

He could be innocent.
He may have the blood of hundreds on his hands and be lusting for more.
We can not know, so we must be cautious.