Earlier quoted context omitted.
He wasn't in the UK.
Extraterritoriality of embassies is a myth. Assange was in the UK when he was in the Ecuadorian embassy. However, the UK authorities could not have removed him from the embassy without violating the Vienna Convention.
Along with the myth of not being "in" country X when you're in airside transit on an international-international itinerary through an airport there.