@Stuart **********
You can do the homework yourself using the WISE app, and not rely on random people.
Set up the transfers and you will see the amounts that would be finally paid into your Thai account.
I did it recently.
Converting to THB inside of WISE to a WISE THB account before transferring to a Thai bank came out marginally better after charges than sending GBP.
However, that's based on just a couple of transactions and hardly statistically significant.
I expect that WISE tries to minimalise the arbitrage and there will never be a significant difference.
Let us know the results....
Just did it.
If you want THB 20k in the Thai account, it will currently cost GBP 452.12 via a WISE THB account, and 451.99 directly to a Thai bank.
13 pence cheaper going directly.
Or 0.03%