I am in the same situation my PCP deal is based on 10k a year but the car has just hit 2 years old and I've done 25k

I didn't know you could change the mileage on the finance deal but imo its unnecessary.
On a PCP deal there will usually be 2 consequences:
1. As one of the options of PCP you can simply just hand back the car, if you have gone over your agreed mileage you will be charged the excess mileage charge usually billed as pence per mile, this will be listed on your finance contract. The only situation where this should be invoked is where the car has depreciated a lot more then originally estimated when the contract was written (very rare from what I have seen).
2. If you are planning on part exchanging the car for a new one you may find that the value of the car has overtaken the estimated residual value leaving you in negative equity. Effectively you have to pay the dealer to take the car off you. Since most PCP deals are designed to leave you with positive equity at the end of the deal depending on how much you have gone over your mileage you make find you are ok, brake even in the worst case.
Either way as dan said, you are probably best to start saving just in case you end up in negative equity or hand back the car with excess mileage.