Just exchange in Thailand rather than in The UK. Thailand has much better rates due to lower labor cost, especially when you visit places like superrich. If you plan to exchange a large sum and you want to get the most out of your GBP, you can drop in a bank in the UK and withdraw 💷50 notes and bring them to Thailand. But just remind you that you want new and clean notes because the old and dirty ones might be refused at an exchange place in Thailand.
Here is the webpage where you can check the rates at superrich:
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And here is the website where you can find the best exchange rates in the UK (and they look like extortion compared to the rates in Thailand)
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