I suppose Britain could end diplomatic relations with Ecuador and then "legally" go in and get him but that could be construed as an act of war (Ecuador could then "legally" arrest and imprison British diplomats there it would seem). They didn't even do that in the time of declared war with German diplomats, I don't believe.
Britain wouldn't turn him over to us if he was charged with murder and was eligible for the death penalty so they like to get up on their high horse no matter whether it's pointed north or south.
There is a law from 1987 that allows the immunity of an embassy to be lifted for short periods. i am not sure if this is in response to the Iranian embassy seige in the 1980's or not. It has not been used very often at all, but it is legal. I think the idea is that it does not allow embassies to abuse their immunity by granting asylum to anyone accused of a crime in the country where the embassy is based in. If Assange is allowed to flee via this orute it means anyone in any country can do the same i.e if an american accuse dof murder goes into any EU embassy they cannot eb handed ove runless the death penalty is not used and unless the trial meets the requirements of the ECHR act, it basicly means other countries laws suddenly become applicable.
I really do not get the idea of claiming asylum in Ecuador when you are supposed to be a champion of free journalism. Ecuador does not treat its journalists well, Sweden however does.