I use
www.airline-network.co.uk
I just done a random search on there using thai air as the airline , using differing flight dates eg..outward 24th , 25th & 26th , return dates anything from say 10th - 15 th jan 05
Prices are usually expensive booking early so i was not surprised to see the whole cost (heathrow-bangkok-phuket ..return) anything from just over £700 - just over £800.
I usually call the number rather than book online nearer the time as for some reason there are a few better deals which never seem to show up in the search engine.
But i did find flights available quite easy & this is only on the 1 airline so it proves there are flights still going which isnt bad considering xmas day is the outward journey & is probably one of the busiest days of the year.
I usually book via this website anything from 2 - 4 weeks before departure & usually get a good deal.
Last april i paid £450 return all in (london-bangkok-phuket) on thai air
Try to bear in mind there are always late deals if you remain patient , there are many airlines to choose from also but internal flights are restricted to only a few companies