He's still got to get from the embassy to the car (remember that the embassy is a two bedroom apartment - it's not got underground parking on embassy soil a car could get into) and then from the car to the plane.
A bunch of the newspapers here (the UK) looked into this at the time and the short version was there was no realistic, workable way of getting him out without the UK co-operating.
Contrary to popular belief, diplomatic missions do not
enjoy full extraterritorial status and are not sovereign
territory of the represented state. Rather, the premises
of diplomatic missions remain under the jurisdiction of
the host state while being afforded special privileges
(such as immunity from most local laws) by the Vienna
Convention on Diplomatic Relations.
