Well, as others have pointed out: Being in the UK actually makes is _more_ likely to be extradited to the US, since when he's in Sweden, both Sweden and the UK would have to agree to an extradition request. The UK is also not particularly well known for fighting US extradition requests. That's certainly not accounting for extralegal extradition, but that's a different pair of shoes. I don't think that being in Ecuador helps against that any better than being in Sweden.
Given that, I have the feeling that he was actually trying to avoid the trial and using the fear of extradition to the US as a convenient excuse. I may quite well be wrong on that, but that's how I currently feel.