After 16 years in phuket the most cost effective way to use pounds in thailand is to bring lots of sterling back to thailand when you visit the UK then exchange in Central Festival for thb. I worked out i was getting about 6-8% more for my sterling that way. No 220thb atm fee and you avoid poor exchsnge rates from the banks to. Ive tried lots of ways and thats by far the most cost effective way