You can do a PCP with your mileage, just need to declare it at the start. This determines the GFV (final baloon payment) amount and consequently, the monthly payment. If you go over your mileage then you will probably face a charge of around 6p per mile (+vat) at the end of the term.
You don't own the car at the end of the term (say 36 months), but if you had a 60 month HP agreement, you wouldn't own the car at the 36 month point anyway. Further, you would probably have a good chunk of negative equity to soak up and be stuck with the car (MOT's, no warranty etc) or have to borrow the neg eq on top of the price of any new vehicle, getting yourself deeper into negative equity next time and so on...
Out of interest, what car are you looking at getting?
JJ
You gotta tie yourself to the mast my friend, and the storm will end.