4 airports: London Heathrow (LHR), London Gatwick (LGW), Stansted (STN), London Luton (LTN). Here is the time to allow if your stopover takes you from one to the other.
Estimate: 45 min to get out, move and clear security again, plus the journey (real distance × 1.3 at an average 40 km/h). Check shuttles and traffic on the day.