It just doesn't sound like a good use of the officers' time.
They've had the guy detained without charge for 500-odd days. Facing some international criticism and generally looking like hypocrites whenever they want to criticize some other country for having political prisoners. The case has been through their Supreme Court and now there's yet another country involved in this international incident.
When I was in London last year there were helicopters circling continuously in some places. If Assange gives them the slip somehow, it's not going to be because the local police preferred to prioritize something else that day.
How many times? This is nothing to do with the US. This is a UK-Sweden extradition of an Australian citizen. If the US DoJ ever gets off its ass and decides to press charges against Julian Assange (they've been sitting on their hands for a couple of years now) it's not gonna do it via some subtle cloak-and-dagger six-country shuffle, it'll just show up with a letter that says "o hai extradition plz".
Is an embassy really obliged to shelter any foreign national who might show up on the run from the law for a fairly prosaic [ahem alleged] crime? If I knock over a gas station can I show up at the Tanzanian embassy, say "Hi, I'm a political refugee" and hang around for long enough for them to process this claim?