First of all, PCP is a hallucinogenic drug that is best avoided.
Secondly if you enter into a contract that is based on you doing something (6k miles p.a.) that you have no intention of doing, then you are going to end up being taken for a (14k mile) ride.
Thirdly, Mercs are better made than BMWs but they don't have as much "pulling power".
Forthly, most finance plans are designed to confuse and overcharge buyers. If you have £x in the bank to spend on a car, you should have a look through autotrader.co.uk and your local paper, view a few cars, have a few test drives and check a few documents and, when you find the right deal, simply pay the money and take the car.