The only slightly left-field suggestion I can come up with (sorry, been away on holiday) not covered above...

If you can convince your driver that Windsor & Eton Riverside is on the way from Legoland to the M4 (it's not that far off), you could get the train from there to Wraysbury (6 minutes) then an Uber from there to Heathrow (10 minutes).

Not sure if that's much better than taking a taxi all the way, but it could save some traffic/money, potentially. That's all I can think of, anyway