Nick here from Shopify. Good question!
I'm thinking your best bet might be to research and ring up/visit a few different banks and see which ones give you the best rates. While doing this it could be worth looking into which banks are the best for the business bank accounts also. I am based in Ireland so it might be different and set up a business bank account with minimal requirements. The reason I'm thinking it might be good to look into both is that they would go hand in hand in my eyes. Switching your personal bank account specifically for this could be a lot of hassle, but if you could set up your business account to go with it, it might make things easier.
I found this article which discusses the top 25 UK business bank account prices that you can read here. I know this might not be an exact answer to your question but hope it has helped and points you in the right direction for next steps.
All the best, Nick