This isn't really a criticism of PCP or any lease or HP agreement. It's just pointing out the difference between buying new vs. buying 2nd hand. Any form of leasing is just another way of financing a purchase, same as taking out a loan from the bank. [The latest accounting standards force businesses to treat a leased asset as if they've taken out a loan to buy that asset, recognising that the lease is just debt by another name]
You could do the same as the lease company if you borrowed £30k or so to buy that BMW 118, paid the bank £396 per month to cover interest and repay some of the principal, then sold the car at the end of year 4. Theoretically, if the lease co' has got their sums right, the value of the car should just about give you enough proceeds to repay the outstanding principal.
You could argue that the 7.9% interest rate is higher than you could borrow elsewhere, but it's not outrageous, and this is basically how the lease co' makes a profit.
The economics of leasing are very simple - your monthly payment needs to cover the interest on the new-car purchase price and also the depreciation of the asset over the 3 or 4 years of the lease. On a new car of say £35k both these amounts will be related to that new price. There's nothing inherently wrong with using a lease to fund a car purchase, but inevitably buying or leasing a new £35k car is more expensive than buying or leasing a 2nd hand £20k car.