Thanks to the excellent Spy Blog, the blog operated by the Watching Them Watching Us campaign for the regulation of surveillance technology, I now know that the controversial Anti-Terrorism, Crime, and Security Act 2001 made it a criminal offence to "detonate a nuclear weapon in the UK without permission". What? All these years it was quite legal to let off nukes - and I never even tried! And - where do you apply for permission?
Prosaically, I suppose the bit about permission is to cover the possibility that the govt. might one day resume tests and do it in British territory, on an island perhaps. Is that form U-235, I wonder?