OP I don't think your being selfish etc. - I'm in a similar position: I really loved oxford on the open day in summer, but gradually went off it after I had applied. I hated the interviews and the atmosphere when I was there, and just assumed I would get rejected. Now I've got an offer, and don't know what to do with it. But at the end of the day, I want to go into research, and I can't help feeling that if I turn it down I will regret it!
Your parents should understand that if you really want to make a career out of your subject, you need to go somewhere you enjoy. It's better to go to another uni than to hate Oxford and drop out in the first term!!!
Anyway, hope you make up your mind.