From an outsider's point of view:
If you ask the P/F to vacate soonest - AND they confirm - and they then do not do so, they have disobeyed your instructions and can be reported for so doing?
In the exchange that you relate, the pilot appears to have agreed three times to vacate early. Once whilst airborne and twice in the roll out?
We can all understand that a longer roll out speeds arrival at the gate but if pilots start ignoring your commands - then they HAVE to be reported!!
I was amused that Nigel's think they own LHR, we have something similar in my field. I am in telecommunications and, despite 18 years having passed since the first Act of privatising GPO Telecommunications into BT .... BT behave as if they still own the lot!