Am I missing something here?
The Swedes can't interview him in London because, er... he's in the Ecuadorean embassy which, to all intents and purposes, is a little chunk of Ecuador. If Assange doesn't want to speak to the Swedes (and the fact that he fled Sweden when they asked him to talk to them implies that he doesn't) then they can't force him to speak to them, at the very least the Ecuadoreans will simply deny any Swedes access to their embassy or their house guest.
Assange won't leave the Embassy because the moment he steps outside he will be, quite rightly, arrested for breaching the UK bail conditions. I'm sure some of the Liberal numpties who put up his bail would also like to have him in civil court to attempt to get some of their money back so expect some civil supoenas to be processed immediately as well.
British police can't go in and get him, unless they order the closing of the Ecuadorean embassy, which they're not going to do, but they also can't just let the little twat walk out of the country, so have to maintain a vigil for the moment the white-haired muppet leaves the Embassy's 'protection' while enduring the continued presence of their fugitive on Russia Today! Fancy letting someone who shows that much respect to your country's laws, reputation and society waltz off to the rainforest - Nope, neither does any home secretary.
So it's nothing to do with lazy Swedes, or incompetent Met police, but everything to do with a self-righteous, self-important asshole who thinks that laws do not apply to him, and a country that is pissed off with the western world seeing an opportunity to willy-wave on the global stage versus the rest of the world (less the Guardian readership) saying 'No, you are NOT going to get away with this. This kind of behaviour is an attack on the very bedrock on which our societies are built'.
I suspect this will go on for a long time, and may well require a change of government in Ecuador to resolve the matter.