HMRC publishes monthly exchange rates on the Trade Tariff service. They are published at the end of each month and cover the following calendar month, for both duty and import VAT. HMRC checks them weekly against commercial selling rates and updates any rate that differs by more than 5%.
HMRC's guidance explains when a rate set in your contract, or a fixed rate, can be used for customs value. Otherwise you must use HMRC's published rate in force when the import entry is accepted.
Your bank or payment provider sets its own rate when you buy dollars. Compare the rate quoted with a published reference rate at the same time. The gap is the provider's margin, on top of any fixed fee.
Sources
A summary of the public sources above, checked when it was published. General information, not legal, tax or financial advice.