Well, what they'd have to do is strip the embassy of its diplomatic status, which they're entitled to do as the host country. Doing it for something like this would be unusual but not necessarily illegal in international law. Since the Brits don't recognise diplomatic asylum, and they don't have to, there's an argument that Ecuador itself is breaching international law by putting Assange out of reach of the British authorities on their own territory (embassies remain under the sovereignty of the host state, contrary to common misconception). Obviously there's a point of principle here, although whether this is the best case for either side to make it on is another question.