Worth adding...
Hoppa buses running a normal schedule on Christmas Day.
Heathrow Express on Christmas Day / Boxing Day
T2/3 to T4 every 30 minutes on 06 and 36
T2/3 to T5 every 20 minutes on 10, 30 and 50
T4 to T2/3 every 30 minutes on 50 and 20
T5 to T2/3 every 20 minutes on 00, 20 and 40