So the car thing could work if he somehow made it to the car? Or they somehow transported the car into the flat?
The car would have diplomatic privilege, but that's fairly limited.
If he could get from a second floor flat to the car the British police couldn't remove him from the car. They could, however, stop that car from going anywhere. They can't enter it but they don't have to give it free passage.
In addition he would eventually have to leave the car to get on a plain. Even if they brought a plane that had similar privilege the British don't have to let it leave and could arrest him on the way to that as well.
So basically he has asylum which means he can stay in that flat for as long as he care to. There is no practical way he could get out of the country and realistically he is going to be arrested as soon as he leaves the flat.
The likely scenario now is he eventually leaves the flat (or the flat's diplomatic status is revoked after a protracted legal battle), gets arrested and detained, gets extradited to Sweden, is charged and tried, either jailed or cleared (more likely jailed given the charges and past statements), then eventually re-extradited to the UK to be charged over skipping bail (maximum of 12 months jail).