Slightly jumbled notes to say the same - use Wise.
Auto convert within Wise - £10.94 on £2000
Spend THB with Wise card: free
Transfer Wise THB balance to Bangkok Bank: 31.70 Baht (79p) per trx, any amount
Convert and transfer: sum of above (eg £2000 => ie, 10.94 + 0.79)
So rarely any reason to just convert - you may as well set up an automated transaction a percent or two better than the current rate, depending on how much of a rush you're in, turn your phone off and forget about it, and Wise will snipe the better rate whenever a peak comes along.
Global Reach (a dedicated broker) just gave me a quote, £20k 6th Dec 5pm gmt+7, 42.59 no fees, so 851,800 (haven't clarified but assume 500 Baht receiving fee would also apply when they SWIFT THB to Bangkok Bank)
Interbank rate, at time 42.7467 (I miss this rate!)
Bangkok bank TT buy at time 42.
*****
, net 845,950 Baht
Wise at time 42.7570, £110 fee (0.5%), net 850,430 Baht
Currency club 849,867
Currency fair 851,923 (assume receiving charge of 500 Baht to come off too)
Remitly is good if you're American.
Looked into a few others, including moneygram and XE, nothing comes close to these. monito.com gives you some quotes.
Alternative currency brokers much like Global Reach are Currencies Direct, TorFX and Moneycorp (but they refused to give me a spot rate without passing all the 'know your client' checks).
So Global Reach better than Wise but only by 900 Baht / 0.1%. They could probably push things though - this was just a spot rate reluctantly shared.
Currency Fair was better than Wise by 1,100 Baht / 0.13%. EDIT: Checked again a few weeks later, amid a sinking market, and on both occasions CurrencyFair was worse than Wise. I also tried CurrencyFair and had a week of paperwork proving source of funds before they'd release my converted funds, by which time I'd lost a fair bit on the rate. My money was from savings, so I couldn't provide a wage slip etc. They threatened to keep the money. Wise always easy.
On CurrencyFair and Wise, you can put a limit order, or automated transaction, that triggers if a rate reaches a given level. These need time and care, but are great for grabbing a rate that is only available briefly due to a momentary spike. With Wise, they leave a THB balance in Wise, which can be spent on the card absolutely free, or transferred to a Thai bank for 32 Baht (fixed charge, regardless of amount).
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If you use this to open your Wise account, you get fee free transfers for your first £500. If I get three people to transfer £200+ in one go, I get £50.