Unfortunately it seems as though there are not any award seats (i.e. ones bookable with Avios) left for that flight. Your best options - in decreasing order of certainty of getting the upgraded ticket - are now going to be:

- Pay for an upgrade by calling BA Holidays and asking them to price the upgrade
- Ask at the airport if they have any upgrade offers [known here as AUP] (see here: https://www.flyertalk.com/forum/brit...20-thread.html)
- Hope for a POUG (see here: https://www.flyertalk.com/forum/brit...-thread-7.html) although in my experience, I never get these on BA Holiday bookings

Just to add, if I had a 6+ hour wait at Heathrow, I would probably pop into central London and get some lunch and/or go to a gallery/museum. 6+ hours would not be too enjoyable in the lounges at T5 no matter what - although there will certainly be people who disagree with me there.