Brexit
My guess is that he's waiting to see how Brexit plays out. A hard Brexit will mean the end of European Arrest Warrants as far as the UK is concerned. If that happens, he can leave the embassy. He'll be arrested for bail-jumping and probably serve a few days in jail, then be let free. He won't have to face the rape charges. Where-as if the UK is still part of the EU when he leaves, then during his stay in jail the warrant will be revived and he'll be off to Sweden.
Since Brexit day is only a few months away, it makes sense to wait. If Brexit is cancelled, he'll have to think again. If May's deal goes through, I've not idea what the implications are, but I suspect he's still stuffed.
Biting the hand that feeds IT
