The correct airport pickup time is not simply “the drive time plus a bit”. Start with the airline’s advised arrival time, work backwards using a realistic road-time range and then add a contingency appropriate to the route.
Work backwards from check-in
Check when your airline recommends arriving and whether a bag-drop desk closes before departure. The relevant deadline is not the aircraft’s take-off time.
Use a range, not the fastest map result
For Heathrow, a planning range from central Andover might be roughly 70 to 95 minutes in ordinary conditions. Gatwick, Luton and Stansted need wider allowances because the route exposes you to more potential congestion.
Tell the operator everything
Give the flight, terminal, departure time, address and date. A local operator should recommend a pickup using experience, live conditions and a realistic buffer. Ask what happens if a serious incident affects the route.
Do not copy a generic time blindly. School holidays, weekday peaks, roadworks, weather and terminal changes can all alter the plan.