Heathrow to Southampton coach: the simplest public-transport option
National Express currently publishes direct Heathrow to Southampton journeys with the fastest trip listed at 1 hour 25 minutes from Terminal 5. From Heathrow Central Bus Station, which serves Terminals 2 and 3, the fastest published journey is longer. The exact time depends on your departure stop and service.
The big advantage is simplicity: you can avoid travelling into central London and avoid moving heavy luggage between rail stations. Current route information shows direct services toward Southampton Coach Station, with some services also calling at the University of Southampton.
National Express publishes Heathrow departure points at Heathrow Central Bus Station for Terminals 2 and 3, outside Terminal 4 and outside Terminal 5. However, not every Southampton departure necessarily follows the same stopping pattern, so use the exact booked service rather than assuming every coach stops at your terminal.