Re: On a serious note
There's no mechanism to do this, unless they pass a special law. The government can't officially tell the police what to do in operational matters and they can't tell the courts what to do at all.
And of course Sweden's investigation has been dropped for lack of progress. Their prosecutors office is apparently forced to act proportionately, and so give up when they can't get anywhere. It appears (though it's obviously hard to know), that Ecuador lied about cooperating with the Swedes, and didn't allow the formal interview at which Assange could actually be charged - only alllowed some sort of limited questioning - hence they gave up.
However as soon as Jules leaves the embassy, Sweden can re-issue the EAW on the one remaining outstanding charge and get him back. So even if he's only held by the Met for one day, before being taken to a magistrates court, fined for bail jumping and deported to Oz - that gives Sweden the time to get him again.
Last time Ecuador tried to get the Foreign Office to talk, they didn't get very far. And that was when the Met were supposedly spending a million pounds a second stopping his daring escape attempts - presumably overtime doughnuts cost extra...
So I suspect this is wishful thinking, give the attitude displayed here by the FCO:
At a meeting last Tuesday between Ms Alban [Ecuador's ambassador] and Hugo Swire, the Foreign Office minister responsible for Latin America, Ms Alban is said to have asked: "What are we going to do about the stone in the shoe?"
Mr Swire's response, according to a source who was in the room, was: "Not my stone, not my shoe."
From what I've read from Charles Crawford (an ex ambassdor of ours), the Foreign Office take a dim view of this sort of thing - he says the job of an embassy is to build as good a relationship as possible with the host country, and be as clear a communications channel as possible - and not deliberately creating embarrassment for them. Which is why British embassies would try to avoid taking in even legitimate cases seeking asylum if they can manage it, let alone this crock of shit cooked up by Assange. So they've no incentive to help, and from what I understand actively want Ecuador to suffer the consequences of having taken this pot-shot at the British system.