He's subject to a European Arrest Warrant. It's not the UK police's job to question him. The procedure in an EAW is that the receiving country checks that the warrant is valid (which is procedural, and doesn't assess the merits of the case). If it is, then they arrest the suspect and hand him over to the requesting country. Julian Assange had every opportunity to argue against this, and was able to take it all the way to the UK Supreme Court. They ruled against him, which is when he skipped bail.
But isn't the fact that they obsess over catching him when he leaves the embassy, over of a claim of sexual molestation that even the "victim" doesn't really support, to the tune of 12 million pounds, a hint that maybe it's not really about this case alone?
Another hint is that Roman Polanski lives openly in France despite being convicted of drugging and raping a child but somehow Assange is much of Europe's priority...
It doesn't surprise me that European countries that are all bound by the ECHR (among other things) would take extradition requests from each other more seriously than they would a request from the US. The fact that they won't render Polanski to the US cuts against Assange's argument.
