Thai Air seem to be doing an offer right now 1900Bt with tax one way new Airport down to Phuket, because Thai are operating out of 2 airports now there are less flights to Phuket from the new airport than when it was only the one airport. Sometimes the last flight is quite early, but right now it is 22.15 but there is a big gap between this and the earleir flight, Check out their web page.
Thai Airways
As has been said before, if you go back with Air Asia and their flight is delayed or cancelled and you miss your international connection it is your problem. If the same happens with Thai, its their problem. (They would usually put you on the next Thai Air Flight which is a direct flight to LHR)