Britain's nuclear warheads could be triggered .......

[tycho102]

The reason a plane could setoff a nuke is the same reason a sub's nuclear reactor could set one off.

If the missile was doused in burning fuel (or the hot slag of a critical pile), the compression explosives could ignite. It wouldn't be nearly as efficient as a full detonation, since there's also a small particle accelerator that works to reduce the necessary critical mass.

It's one of those really funky things that Phd's are paid to think about whenever something goes to the feasibility phase. There are contingency plans for one of our spy sats going down in Iran or North Korea, one of our nuke subs suffering a meltdown in Chinese waters, and the Space Shuttle having to make an emergency abort onto the African continent with classified gear aboard.