Whether or not it is legal would be pretty much irrelevant, I'd think - it would be something every regime that dislikes Britain would pay close attention to for the purpose of pointing to the moment they decide they want to pick someone up from a British embassy.

I can't imagine Britain would be willing to risk the integrity of its own embassies in that way over Assange. It sounds like sabre-rattling, nothing more.

Consider that they did not even try to enter an embassy over the killing of WPC Yvonne Fletcher by someone at the Libyan embassy - a murder that achieved infamy in Britain. The Metropolitain Police laid siege to the Libyan embassy for 11 days and the government severed all diplomatic ties with Libya. But they did not try to enter.

If Thatcher was not prepared to do it, I very much doubt the current government would be willing to deal with the fallout.