[Windflier]

...isn’t a car with diplomatic plates the same as sovereign territory? Like the embassies and their grounds?

To my knowledge, yes, they are the same.

Assange will need a diplomatic/military escort all the way to Australia, if he's to stay out of prison.

[oldvirginian]

“To my knowledge, yes, they are the same.”

Then an Australian diplomatic car picks Assange up at the Ecuadorian embassy and drives him directly to an Australian government owned plane at Heathrow for the flight to Australia.
All properly overseen and escorted by Australian diplomats and embassy guards of course.
Don’t trust the Brits not to make a clusterscrew of the situation though.