I live in Thailand, and you're actually better off withdrawing your money here in BAngkok. There are 2 different exchange rates, both official, one onshore on one offshore. If you are buying Thai baht, you're much better off buying it here, difference in rate is c4baht to the pound, so a fair bit if you are spending a lot.
Best bet is the currency exchange booth in the airport baggage collection hall, fast, efficient and friendly, plus you will have a wait for bags anyway. If you are just transiting in bkk, then I think there are the same booths in Phuket and samui airports.
I think if you withdraw from an ATM though, you get the offshore rate because the conversion is done in the uk, same as with credit cards. So cash from a forex booth is your best bet, there are a gazillion of them in Thailand.
In the unlikely event you get stuck and need help in Thailand, feel free pm me, have already rescued one mnetter who was delayed with flights, so offer is there!