National Express are running an hourly Christmas Day schedule between Victoria Coach Station and Heathrow.
Fare is £10 on-line, nearly double the usual fare.
Only seems to be running to the Central Bus Station though so if you are using Terminal 4 or 5 ??
As far as I know this is the only public transport available on Xmas Day if you are using Heathrow.
Don't seem to be running to Gatwick, Stansted or Luton though.