Heathrow sits about 28 km west of central London. Off-peak the drive takes around 55 minutes; weekday mornings and Friday evenings can push it past 80 minutes.
Your driver meets you inside the arrivals hall of your terminal with a name sign, and the fare already includes parking and the congestion charge where it applies.
M4 into the city; the last stretch through Hammersmith and Kensington is the slow part.