PCP works out more expensive no matter what if you dont pay early due to the added interest. Take the extra discount they give, its usually £1k and pay the balance off after 14 days. Sometimes you have to pay 3 months worth of interest but its negligible compared to the discount given.
I tried PCP once on a Merc a while back. They take money off for absolutely everything, including wrinkled seats (wtf?). Apparently, it was wrinkled more than it should have been, i just told them to bugger off, bought the car with the baloon payment then sold it privately!
I dont think its worth the hassle though, I prefer getting PCP and paying it all off in a month or just buying with cash. Does work out cheaper buying with cash (although for some cars it doesnt if it depreciates alot, usually the high end models like the LS). Best to work it out first.