Well there's your problem - lookers! lol!
The only way you can do it without worrying about the hp is to do one of the following:
1. Sell the car for the same amount outstanding on your HP (highly unlikely)
2. Go to bank, get loan - enough to pay off HP and buy new car, go somewhere better and cheaper than lookers who won't rip you off, buy new car!
Or, try going to a main dealer and see if they can finance the new car and the old car together. I know the dealers I got my new car from managed to do it so I'm technically still paying off the outstanding loan I had on my old car in with the finance on this one. (If that makes sense)