You are financing the whole amount minus any deposit, so you pay interest on the whole amount (£11,692) it’s just that a large section of the amount gets offset to the end as a GFV (or Optional Final Payment) is a better phrase IMO.
8.9% apr is the problem, if you had the money in a savings account it would be paying 0.1% so these deals are for people without either the money or the credit rating to get a cheap personal loan.