Re: "A third country might offer a new couch"
But he'd still have to get from couch A to couch B, meaning he could be nabbed en route. Although the Ecuadorian embassy is now no longer under 24-hour surveillance by the police.
If he could go roof-to-roof by helicopter they wouldn't get a chance. You'd struggle to force a helicopter to land in Knightsbridge without copping massive amounts of flack from... well everbody. It would be dangerous for everybody concerned, including innocent pedestrians and bystanders.
The most you could do is watch as Assange flew from one embassy roof to another and then arrest the pilot when they landed at an actual airport/helipad.
Of course if that pilot is a foreign national on a diplomatic passport brought in specially for the job, then there's not a whole lot they could make stick - pilot goes home, continues flying in home country, is well paid off for their services.