If the bank account is Euro then you won’t be able to tag a transaction to a GBP invoice, you would need to log the payment from the invoice itself. This may create a duplicate transaction, so you would then need to delete the untagged duplicate.
When I go to log the payment from the invoice It wont allow me to select the bank account for which the transaction is in. I have two bank accounts and it will only allow me to select the current account bank account but not the other bank account that has been set up.