Ive been in Thailand for 16 years and have always looked for the cheapest way to exchange my own currency (pounds). After 14 years i realised i could save about 5-6% by bringing my own currency then exchanging it here. You save the 220thb ATM fee plus you get a better exchange rate than your own bank give you via thai atm (avoid tourist area exchange kiosk where rates will be poor) plus your bank also charge an international withdrawal fee. So if you spend 2000 euro a month in cash youll save perhaps 100-120 euro a month. Obvious downside youll be in charge of a big lump of cash which could get lost or stolen.
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