The Accesrail website states:
First Great Western now provides, thanks to AccesRail, its first air-rail intermodal solution. This allows passengers to go to/from Heathrow or Gatwick Airport to their First Great Western destination of choice.
Passengers receive an intermodal ticket that covers both their flight and rail journey. Depending on which airport the passenger travels, train connections are slightly different.
As a passenger, how do I purchase such an intermodal ticket? Accesrail does not seem to sell them. There is rail-fly.com but that seems to cover only flights to Singapore.