Enough of it to stop anyone providing a thrid party with a bomb to smuggle it into the UK to detonate.

For a better explanation of how this works, read up what Jacques Chirac's take on it is. The french Navy have already reconfigured one third of the warheads on their missiles to counter exactly such a threat. They did that as soon as Iran started their latest round of games with the IAEA and the NPT. Mind you it must have come as one hell of a shock to France when it was discovered how far Libya's nuclear programme had progressed (Courtesy of Dr. A. Q. Khan).