The UK has resisted on more than one occasion. That the Assange extradition saga has lasted this long, is a prime example of that. Other cases before them has shown that the UK courts have a spine, and UK politicians, while they have far more power to override the courts if they're willing to do so openly before parliament are too timid to let things like the kidnapping and renditions Sweden were complicit in happen.

But in any case, the issue isn't whether he was right, but whether he believed he'd be safer in the UK than Sweden, and I think the Swedish prosecutor did a whole lot to make him worry that something fishy was going on. I've written more than once over the years I think she was acting out of political/ideological reasons (specifically, she had a long history of fighting for much stricter treatment of rape cases) rather than due to US pressure, but the net effect was a whole string of incidents around the case that'd easily look mighty suspicious for someone worrying about the US trying to get them.

I don't know whether or not he was right to fear it, but I'm surprised he was.