I have Gap on both our cars and the are on pcp, its just not worth the risk!! Both policies were bought through the dealer, at first their price is silly but if you are up front and just tell them that you will buy it but at such a price they normally do it. Porsche wanted £700 and I got it for £399, I am sure I could have got it cheaper but I like having it done through the dealer.
The m5 deal is a lease so to hand it back early you will have to pay the remaining monthly left on the agreement. So if you were 12 months in you would have to give them 24 months payments.