Assange currently has the run of the Ecuadorian embassy in London (roughly the size of a large flat) but the minute he leaves that (for instance to try to get to an airport) he'll be arrested for breaching his bail terms.
Even if he's granted asylum (relatively unlikely) that won't change unless they make him an Ecuadorian diplomat with the diplomatic immunity that confers. That would however be essentially unheard of.
Assange isn't going on the run anywhere.
Even if that weren't the case he's got to get out of the car to get into the plane at which point he's once again on British soil and can be arrested.
Plus really, how much effort at the Ecuadorian government going to go to to piss off the British, American and Swedish governments? What they're doing now is following procedure, what you're proposing would be far more active.
I'm only half joking. ;)
