From my understanding, most PCPs end with three options - (1) you buy the car for the GMFV, (2) you hand the car back and (3) you use the car in a part exchange for another one and any equity in the deal is all good for you. The mileage (and condition) becomes an issue if you try option (2) - anything over what you agreed and anything more that "reasonable" wear and tear (my words) can incur cost as part of the hand back.
I believe most PCPs are now regulated under the consumer credit agreement which means they also have a hand back clause when 50% of the outstanding finance has been paid - worth checking out to make sure you fully understand what you're getting yourself into!