Because the CIA can't use the same techniques as British intel can. Like pulling a trigger on someone to become a "made man".
The US has strict laws against that kind of thing for our intel folks. The UK doesn't. In fact, some of the techniques that the UK used against the IRA were pretty brutal (but justified).
/john
"From nuclear weapons the threat is very, very slim," said Gary Samore, a former US National Security Council member.
What's this supposed to 'really' mean?
:-(