Yes, As Jimbo says YOU are supposed to declare it on leaving the UK,IF you dont and it is found in your hand luggage,YOU will face interigation by Customs & Revenue to see if it has been legally obtained and tax paid,Because they could construe it as WASHING money ( Money Laundering ) So a letter from your UK bank,or Building Society would I think suffice.I Did bring out 20,000 gbp to buy my MY WIFES

House when we first came out here,But at that time I NEVER declared it as Laundering was'nt so rife,I Later found out that I could of had it confiscated,as I did'nt get a letter from my Building society,saying that it was LEGAL money from the sale of my UK House......I Certainly Would NOT Declare it at Swampypoom,They would all be running around like Headless Chickens,But go straight to a MAIN office of your chosen Thai bank and either Deposit it,or get a safety deposit box put in until the exchange rate gets better,& just change it as you need it......
