@Br***
yes if you specifically plan to send the money back after, that is the reason to use a FCD, so you save (a little) on forex costs. These days you need to leave it all there for 5 months and half of it there the rest of the year though.
"If the exchange rate goes up 1 baht" - but equally it can go down one baht, and you don't know which way it is going to go. The long-term trend against GBP is that THB has been getting more valuable, not less.
If you actually plan on living in Thailand, rather than this being a temporary holiday where you will go home after, you are probably better off having your money in the local currency.