Downing Street claims that, under British law, a week advance notice would be enough to strip the embassy from diplomatic immunity. However such an assault would be almost unprecedented.
For example many North Korean refugees use the principle of (relative) extraterritoriality of embassies to gain asylum after reaching China. China does not assault embassies for that reason. Similarly Honduras did not assault the Brazilian embassy when legitimate President Manuel Zelaya used it as safe platform to gain back access to the country after the military coup.
Recent history is full of examples of not assaulting embassies on political refugees' crisis. So why would the United Kingdom circumvent this diplomatic principle in order to arrest someone who is accused of sex without condom?
This kind of attitude only adds weight to the claims by Julian Assange that he is the victim of political persecution and not a common case of sexual misdemeanor.
As you may know the defense of Assange claims that neither Britain nor Sweden nor much less his home state, Australia, guarantee his rights and that all them will probably end extraditing him to the United States where his human rights would be severely violated on emergency laws, with even risk of summary execution, never mind torture.
Ecuador finally granted political asylum to Julian Assange after procrastinating on the matter for two months. The formal reason is that Assange risks being extradited to the USA, where his human rights would not be guaranteed.
The United Kingdom has previously extradited even its own citizens to the USA, state with whom Great Britain has an unsymmetrical extradition treaty (the UK extradites its own citizens to the USA but not vice versa) which effectively makes it a vassal state at the same level of Colombia.
It is unclear if Sweden or Australia could extradite Assange to the USA as well.