If he leaves he'll be arrested, if he manages to get in a diplomatic car the car will be stopped with Assange in it and there'll be a stand off. If Ecuador tries to make him into a diplomat the UK can simply refuse to accept that he is a diplomat. And if they try to use a diplomatic bag then the UK will claim that it's being used for improper purposes and block it. IMHO Assange is stuck there until a deal is worked out.
The granting of asylum is just one step in what will continue to be a tedious soap opera.
My interpretation of the letter that the FO sent to Ecuador is as a statement of a negotiating position given that the FO got wind of the fact that he was going to be offered asylum.