From what I can make out, the UK reminded Ecuador of the Diplomatic and Consular Premises Act 1987. In summary, embassies have their status only while the secretary of state recognises them as such, and diplomatic premises need to be used exclusively for diplomatic purposes. The secretary can unilaterally cease to recognise them as such if he thinks international law would allow, it and is supposed to take some prescribed factors into account, including safety to the public.
The embassy isn't foreign soil or anything like that, regardless of popular opinion.
In this case, based entirely on probably uninformed speculation, I guess the argument goes that the embassy has a man who is wanted for assault and contempt of court and — allowing for the fact that he doesn't have any sort of immunity — is planning surreptitiously to transport him through British territory, itself a crime. So the secretary of state could have grounds to conclude that the Ecuadorian embassy is being used as a property in which to plan the commission of crime rather than for diplomatic purposes and revoke its status. After that the police are obviously free to enter.