[dailyrorschach]

It's somewhat unprecedented, but has happened in the past. http://en.wikipedia.org/wiki/Diplomatic_mission

It's important to know that as the article points out Embassies do not enjoy full extraterritorial status, and contrary to popular belief are not sovereign.

The practice of the host country entering only with permission is based in the Vienna Convention on Diplomatic Relations, and a violation of this would likely have no real material effect. That convention also requires the host country to protect the embassy and other rules. It is unlikely that the UK's allies would object, and even more unlikely that the country in question here would mount any serious diplomatic or military response.

As always power benefits the powerful.