Cite for the claim that the UK is obligated to pursue people accused of crimes in foreign jurisdictions.

No, the government is obligated to uphold the UK law, for example it is obligated to seek the arrest of those who have broken the UK law. Assange is accused of breaking the conditions of his bail. The government can't choose to apply the law differently to different people (an established principle of common law). It is not absolutly obliged to pursue people accues of crimes overseas, but there are various treaty obligations incorporated into UK law.

Also the arrest warrant is for rape. Say what you will about politics and leaks and all that, but the rape doesn't leave office

@DavidGrinberg Stop spreading disinformation. Assanges UK arrest warrant is for failure to appear at a UK court. His Swedish original arrest warrant was for questioning on an alleged sexual assault, an investigation now discontinued.

While I agree with the principles stated in the answer, it fails to answer why the UK has spent so many resources in what, from a mere legal point of view, is not that big of an issue. I bet there are few investigations where the only imputable crime is failure to appear in court that involves continuous vigilance of a location for years.

@SJuan76 - Yes, it is a big issue. He fled on bail therefore he is a fugitive. What happens when the government/law enforcement walk away? Then every single alleged criminal will hide out for n years without consequence. Is this how we do justice? By just giving up? To moot your "so many resources" point, law enforcement is not a business. It doesn't work on profit and loss. So whilst we have an economy that can sustain law enforcement and the justice system, he should be relentlessly perused and held accountable for his actions.

@ggdx there is a big way between "continuous surveillance for several years" and "giving up". Many wanted people are not actively searched for by the police (or they may try from time to time) but they still end caught when they get stopped by something else and their name shows up in the computer. And while police does not work on profit and loss, they do have limited resources and have to manage those limited resources to get the best results. It is not as if they are unaccountable if crime rises or the number of criminals captured is low, isn't it?

I've added a little on why the case is exceptional. It is not common for the police to know where someone is, but be unable to arrest them.

@Sentinel An alleged sexual assault that he was highly suspected of committing, and there is evidence to suggest this wasnt the first time. Also... how does that change anything? He broke a law that has nothing to do with politics. My point still 100% stands

@SJuan76 The key is that the police knows where Assange is. There's a vast difference between actively searching for someone, and ignoring a fugitive simply because he can't be physically reached right now (a condition that could change at any moment). The police wouldn't spend much time searching for a petty thief. On the other hand, an officer who sees a pickpocket is not empowered to turn a blind eye simply to manage limited resources (i.e. his paid time). Likewise, Assange is right in front of the police - he's just almost uniquely pushing the envelop.

@Semaphore Diplomatic issues apart (give them the importance that you want), I agree that the fact that Assange's location is known and that it could be understood as a provocation may be a good reason to use a different approach. But that same argument invalidates the idea that it is just regular police work and that there are no more issues at hand (which was what this answer said originally and what ggdx wants to believe). Oh, and BTW, a police officer may certainly chose to turn a blind eye on a pickpocket, specially if he (as a resource) has more pressing issues at hand.

@SJuan76 I'm not saying it's a provocation, I'm saying it's dereliction to ignore a fugitive when you know his location, vs not searching for one due to lack of resources. As for "more pressing issues" - if whoever is monitoring the embassy sees a murder happening, they could temporary deal with the more pressing issue at hand too, so the situation is identical in that regard. That's not the same as turning a blind eye to a crime happening in front of you simply because hypothetically you could be investigating a more serious case, which is the point I was trying to make.

@DavidGrinberg Be a little more cautious with your phrasing. If you are posting such allegations under your real name, remember that the internet archives are difficult to delete. In my opinion there is nothing certain here, and it is best to keep an open mind.