"On my way to blow up the plane (I'm a member of the Taliban)."
British man, but - going by the name - presumably of Indian (?) heritage.
The article doesn't give his actual background (which seems like craven reporting), but crap joke or not you don't get to make those comments as a young British Asian male in a "private" group chat (neither are the number of individuals in the group described) in that context and not expect there to be repercussions.
I'm suspicious also of the idea this was intercepted by gchq, it's quite believable that's a cover for somebody in the group being (rightfully) worried and doing their civic duty.
Perhaps it's an overreaction and security overreach but there's more complexity than indicated in the simplistic headline here.