Journal uploads are available for multi-currency entities whose reporting currency is in Euro, however for their GBP bank accounts, we cannot use the journal upload function and so need to update transaction by transaction. For one entity this totaled 1500 transactions.
You can only journal in the base currency, any other bank accounts cannot be journaled or imported. As a summary you can only post a journal when the bank account and base account are the same.
The only workaround that Sage suggested was to enter these in the base currency and manually change them in the program or bank transfer