Once he's in Sweden, how does a UK court have any jurisdiction? Anyway, the UK already wants to arrest him and hand him over to Sweden, given that they've been staking out the embassy for several years now.
Imagine if the London police had've spent all those staff-hours chasing domestic rapists. I reckon they would have nabbed at least one that they wouldn't have otherwise, which would bring them up to the same level of arrestees detained as all this effort spent on one guy (who hasn't even been found guilty in any court yet).
The idea that "they wouldn't do it; it's all too politically hard" discounts all the resources they've already spent because of politics.