According to Jean-Jacques Dethier, a development economist at the World Bank:
Plot: Alec Trevelyan (Sean Bean) wants to use an electromagnetic pulse from a nuclear weapon to bring London to its knees and destroy the Bank of England, but not before electronically stealing millions of pounds from the Bank’s systems.
Plausibility: First of all, wouldn’t destroying London and the Bank of England render the pounds you’ve stolen largely worthless? “Not exactly worthless, but close,” says Dethier. Would you be able to convert it? “It’s actually very hard to convert huge amounts of something, which is a problem the Chinese now know well with all their American dollar holdings,” he says. So Trevelyan would have to spend all those pounds in the one country that’d take them: Britain. Whose economy he’s just destroyed.