How will the bank know which account to debit?

The bank will detect that a transaction has been made in a foreign currency and check to see if there is a corresponding currency current account connected to your Debit Card. If this is the case, the transaction will be debited against the relevant Currency Current Account. If not, the transaction is authorised against the default sterling Current Account and currency conversion charges will apply in the usual way.

NB – please ensure that any pending transactions have cleared against your Currency Account before de-linking this account. If the debit card has been de-linked prior to clearing the transaction will clear against your GBP account.