Hi Chris. You probably have been ripped off, but unless you have anything in writing to prove that you told the salesman that you are doing 25,000 miles per year and he said that a PCP at 8,000 miles per year would be suitable, you’re not going to be able to win that argument. The salesman will swear on a stack of bibles that he never said such a thing and would never do so.
Your car is presumably worth less than your finance settlement figure. Basically, you can either sell the car and take a loss (the amount you owe the finance company is more than you will get when you sell it), or give it back at the end of the agreement and pay the excess mileage charges (these whould be spelled out in your contract, but are usually about 10p per mile, which is £100 per 1,000 miles).