A generous tip usually covers slight excesses (at least
In London) . Though I have never been more than 5-6 miles over the allowance though.

In London emirates subcontracts it to a cab agency near LHR.
I don't know about the situation in MAN but it's usually the same in most places I have been ( ie sub contracted )

If you book your chauffeur drive using the emirates website then the contractor usually calls you on the day before at the contact number that you have given @ the emirates site and them you can enquire about whether there is any excess