Unlimited amounts. Why would you want to take 25K cash? Let's hope you don't get mugged or lose your baggage at the airport...
Currency restrictions
Foreign visitors may bring in an unlimited amount of Thai currency. For travellers leaving Thailand, both Thais and foreigners, the maximum amount permitted to take out without prior authorisation is THB50,000 per person or, if visiting a neighbouring country, THB500,000 per person. The import and export of foreign currency is unlimited.
well the exchange rate is normally beter whilst in Thailand and taking cash will mean no transfer charges but obviously as you have mentioned it is risky taking all that cash
You are supposed to declare it to UK customs on leaving. They CAN ask you to account for how you came by it and seize it if they are not satisfied with the answer.