You can either use market rates at the time (which isn't too much hassle if you're looking up rates for 2 receipts on 2 different days, but looking up 30 days of exchange rates is a problem) or the HMRC's exchange rates (these seem to work out a little better if there's lots of fluctuation as well).
https://www.gov.uk/guidance/foreign-currency-transactions-vat-and-tour-operators
You can either use market rates at the time (which isn't too much hassle if you're looking up rates for 2 receipts on 2 different days, but looking up 30 days of exchange rates is a problem) or the HMRC's exchange rates (these seem to work out a little better if there's lots of fluctuation as well).