Well, whilst I am not sure about the distances and limitations for chauffeur drive in LGW I can share my experience in FRA.

If you stay within the mileage limitation there is no problem and it is 100% free and as soon as you pass that mileage the driver (since chauffeur drive often seems to be done by contractors) will charge you for the excess mileage. In FRA that's an additional 1,40EUR/km (approx.) which turned out to be way more expensive than taking my own car so I did not opt-in for the chauffeur, because I'd only pass the limit by about 10km.

You can make a reservation for chauffeur drive on the EK website or simply by calling them. If I remember correctly you have to book your chauffeur no later than 24hrs prior to your departure. I suggest you call EK or Skywards and ask what extra charges will apply beyond 70 miles and figure out if there are any better/cheaper solutions.

I hope that helps.

UAE777