If you take PCP you only have a limited driving "allowance". You can only have your car serviced by the place where you bought it. If there are any marks or scratches on it then that can have a bearing , so I believe.
You have the option to set the driving allowance, and even if you go over that, it will only have a marginal effect on the trade-in value should you choose to roll-over onto a new PCP deal.
You are free to have the car serviced anywhere you like, unles you know of a dealer / make that are enforcing this? Any I've looked at have no restriction on servicing.
The T&Cs will allow some scratches and wear and tear, though full depth scratches over a couple of inches will have an affect as will bad dents or damage. Again, only affects the value if you want to roll-over onto a new deal.