Assuming it the works the same as from the US, transfer GBP, and have the bank do the exchange to THB here.
In the US my bank charges a $45 fee, which makes Swift a better value than Wise and whatnot for transfers over about $10K. Small transfers Wise or some-such is often better.
I always say it's for real-estate purchase, regardless of what it's for.
On larger transfers, the bank always calls me, lets me know what the rate is, asks if it's okay of if I want to wait.