I wonder: if these are essentially just a “paper trail” for deposits made at the BoD by issuing banks, why bother with a physical banknote at all? In the case of an insolvency of a bank, the bank’s notes would still continue circulating, after all.

Somewhat vaguely related to this, not all bank notes in the UK are made equal. Good luck getting someone in England to accept an Ulster Bank, Danske bank, or Bank of Ireland note, despite them all being pound sterling notes issued in the UK worth the exact same amount.