It should be
Dr Interest (P&L)
Dr Finance liability
Cr Bank
Is split payment is not an option you can allocate whole payment (including interest) to Finance liability account and then do a journal for interest element
You’re doing nothing wrong - when you create a journal where one of the lines posts to a bank account nominal code (12xx) that will create a pre-tagged transaction in the bank account to match. If you have a bank feed then this will be a duplicate for the untagged transaction that came from the feed, this is unavoidable but you can simply delete the untagged one from the feed leaving just the locked one that was created by the journal.
Instead of manually creating a journal you could tag the transaction in the bank account as “something not on the list”, and then choose the option to split the payment across more than one code. What this actually does behind the scenes is exactly the same thing - it creates a journal and then deletes the original bank transaction.
The third and possibly simplest alternative given that your bank account and the “finance company” are both 12xx bank accounts, would be to tag the money out of the current account just as a bank transfer to 1254 VW FINANCE, and then manually create another money out transaction on the VW FINANCE bank account for the interest amount and tag that (via “something not on the list”) to 7904. You can even create a tagging rule for this so whenever you create the manual HP interest transaction it auto-tags to the right nominal for you.