@Graham ****
there is another option. You take the 800,000 THB out in cash Thai Baht. Then visit Superrich or SiamExchange in Bangkok or any reliable money changer and order from them GPB equivalent to 800,000 THB. You can take out unlimited foreign currency out of Thailand. Amounts over 15,000 USD must be declared at Thai Customs. You are NOT allowed to take more than 50,000 Thai Baht out of Thailand - beware! Third alternative: ask your Thai bank to exchange into GBP, and take the GBP out of Thailand in cash