Travelcards are available from the machines or ticket offices at Heathrow underground station. It's probably best you get the Heatrow Express from T4 to T1 which is free for transfer between terminals and then walk to Heathrow underground station T1,2 &3 because T4 underground station is closed at the moment. I really do think you should go to central London rather than Staines or Uxbridge, there really is nothing there to do.