Yes it will be converted at the UK rate which is always a lot worse than the Thai rate.
Not only that but you will also be charged a fixed fee by the Thai ATM, which I believe is 100 baht (2 pounds 50p) and also a fee by the UK bank for using your card abroad. The Uk fee varies depending on which bank. The Thai bank's fee is payable, irrespective of amount, whether you draw 10 pounds or 300 pounds.
I don't know the fee that the nationwide charges. Even banks that do not charge in the UK often charge a fee when a debit card is used abroad, and the UK fee is often a % fee and not fixed.
Therefore as you need to draw approximately 12000 baht when you do withdraw, make sure that you go to an ATM where you can draw it in one go. At least that way you are minimising the Thai bank fees.