I live and have lived in Thailand for the past 14 years. The £ has had a very bad week and today at 11am Kasikorn Bank ( formerly Thai Farmers) was offering an exchange rate for travellers cheques of of 45.1THB for 1GBP, Siam Commercial Bank was selling at 44.9. Siam Commercial charge 3% commission for travellers cheques whilst Kasikorn only charge 33baht per cheque . If you are going to use an ATM machine over here you will be charged 150 baht for each transaction. All banks charge this. Irrespective what your UK bank may offer you will have to pay this charge e.g. if you withdraw 5000 baht you will be paying 5150 baht. The best bank for exchange rates is Kasikorn and unfortunately when you exit Immigration at Bangkok you will see Siam Commercial exchange booths which offer very poor rates. My honest advice would be to change a £20 note ( no commission charges on notes) on arrival at Bangkok and wait until you find a Kasikorn bank.