In a PCP you're buying the car. As such the entire value of the car is financed. If you then don't buy it after three years, the dealer gives you a set trade-in value up front.
It's more expensive than a lease because you pay interest on the whole purchase, however, you have more rights with it. After you pay off 1/3rd or so you legally own the car (and can't have it repossessed), I believe, and after you've paid off half you can back out free of charge as long as the car is in good condition (see VT). Furthermore, you know exactly how much the car will cost you to buy at the end of the PCP. You can also pay a PCP off early, and the interest will go down upon doing so, similar to paying off a mortgage early.