The forceful entry of British authorities into the Embassy of Ecuador would not only be illegal but would also set a frightening precedent, putting British embassies and other embassies around the world at risk.
Having said this, what are the remaining options available to Britain?
Under international law, Britain is under no obligation to give Assange a safe passage out of the country since he is not an accredited diplomat. Even if Ecuador goes as far as to give him a diplomatic passport, he would still be subject to arrest the moment he leaves the embassy premises, since only diplomats accredited by Britain would enjoy diplomatic immunity there.
British authorities can continue to surround, but not enter, the Ecuadorian Embassy on a 24-hour basis to ensure that Assange cannot leave undetected. It is true that Assange could not be arrested if he somehow gets in a diplomatic vehicle. But this is assuming that he could get himself inside a diplomatic vehicle before leaving the embassy premises, which is unlikely, since the Ecuadorian Embassy in London does not appear to contain a driveway or a garage. Even if Assange somehow gets into a diplomatic vehicle, he could be arrested immediately upon leaving the vehicle.
It has been mentioned that a helicopter could pick up Assange from the roof of the embassy and fly him straight out of Britain. This option would not work, since the British government controls the relevant airspace and could deny permission for a helicopter to perform such a function, or could force it to land and arrest Assange upon landing, since the helicopter would not have diplomatic immunity. Any attempt by Ecuador to get the helicopter diplomatic status could easily be denied by the British government.