It sounds like you’ve logged the prepayment in GBP, which you can’t then use to pay down invoices in a different currency. I suggest you delete the bank transaction and then log the payment from the supplier side instead - select the checkboxes for the outstanding purchases in the purchase list, “update as paid” and select the “allocate one lump payment” option. Now you should be able to enter the correct GBP and foreign currency amounts, and this will create a new pre-tagged bank transaction in your bank view.
Basically, from the bank tagging screen you can only allocate payment to invoices or prepayments in the same currency as the bank account, if you want to log a payment or prepayment that involves a currency conversion you have to do it from the invoice(s) or the supplier control panel rather than from the bank.