I'm not inclined to hunt down the posts that clarified it. But, withdrawing, OTC, incurs its own charges, which runs around 1%.
With TCs you pay before you travel with costs and then normally get charged for changing into the local currency. When using your debit card at an ATM
Unfortunately, more often than not, it is a thoughtless formality. Many will offer up a wai, then stab you in the back.
Unfortunately, with ATM usage, most UK institutions also add a substantial charge.
This is one of Thai customs that I wish will endure! And I think it will.