Because he skipped bail, which is against the law.

They would arrest anybody who did the same thing.

Do you think they spend as much money on less politically noteworthy bail skippers?

On a bail skipper "hiding" in plain sight in the middle of London? Absolutely. Why is how much money "they" spend even relevant? He skipped bail. He should be arrested and face the consequences just like anyone else in the same situation.

Less politically noteworthy bail skippers generally wouldn't have the connections/money to avoid bail by hiding in an embassy for years, but I would say yes.