The Brits have already warned Ecuador that they will come in after him if they do not surrender him “soon”. There is precedent, they busted into the Iranian embassy in 1980 to extricate the killer of a British policewoman.
Before they do that the should get all the Brits out of Ecuador. Rafael Correa is pretty crazy. I don't doubt for a moment that he would seize the British Embassy and hold the Ambassador and staff. Countries like Venezuela and Argentina would back Ecuador. Britain needs another way to finesse this IMO.