I used the Zipcar flex to come to LHR last week .. we were actually flying out of T3 .. so was expecting to have to get the bus to T5 and then drive back as this Avis site is I thought only for T5 , but the shuttle bus guy said no problem and dropped us at T3 ... I wouldn't count on that being a regular thing, but worked for me last weekend !

I will not be using Zipcar on the return as it's too much faff .. agree with the comments above that it's fairly likely that the move off airport will kill the service over time