Neil Williams wrote:

Or Connect to Hayes and Harlington and then the bus, or SWT to Feltham
then the other bus (though I'm not sure if the latter is valid on a
Travelcard).

Yes - the 285 is an ordinary London bus route (though travelcards/bus
passes were also valid on its predecessor, the T123). It's actually a
pretty good way to Heathrow from most of South London.