It might be well worth your while (and peace of mind) to toss the FR ticket and buy a new one (it may be cheaper to buy r/t than o/w on FR) for the day before. FR apparently will let you change flights for a GBP 15/EUR25 change fee + difference in fare if any (no refund if it's lower).

FR uses a remote terminal so you'll have to take a tram to the main terminal. It may not be a mile but it's far away.

A chauffeured car is going to run at least GBP 50 (that's the rate into/our of the City/financial district).

May I ask where you're flying from and on what dates? It'll help us FTers sugges alternatives.