The UK can revoke the embassy's status any time it feels justified under the Diplomatic and Consular Premises Act and then enter without permission - there would be diplomatic upset, but no legal consequences if it did.
We passed that act because of the Libyan embassy and the murder of PC Yvonne Fletcher. Actually I don't think we'd have even used it then, because (from memory) they only fired out of the window a couple of times and stopped - they weren't a continuing danger to everyone. But the idea was to have the legal power to deal with the situation - plus we also had the Iranian embassy siege and what would we have done had Iran refused us permission to storm the place and save the hostages?
However that's a provision of UK law. I don't believe it's (directly) derived from anything in the Vienna Conventions. So it's very much the nuclear option. If we ever use it, then we've massively reduced the protection that the Vienna Conventions provide to our embassies around the world. Because some dodgy government will always be able to invent a national security reason to do the same to us. In the case of Libya allowing a diplomat to shoot out of the windows it could be justified - to get hold of Julain Assange - no way! He's not worth the increased risk of death/kidnapping to our diplomatic staff - but people randomly shooting around the streets of London would be.