Your parents are wise.
I am 26 years old now, and have owned a few cars since passing my test aged 19 (Ford Fiesta 1.1, Hyundai Accent 1.3, Triumph Herald 1.3, and my current car - a Proton Satria Neo 1.6)
I bought my Proton on finance after the rear diff went on my Herald, and I didn't have the direct funds available to fix it - finance was my only option to get a car at that time, and I bought the Proton as it was the prettiest car I'd ever seen and the lure of that 115bhp was enough to seal the deal for me. My parents told me not to do it, but supported me when they saw my determination and my dad even arranged for a mechanical inspection on the car for me.
4 years on, I'm older and wiser and would never recommend taking finance on a loss. You'd be wiser borrowing to invest in something that will give you a return, rather than borrowing for a little fun. In my experience, that power gain is never enough - it leaves you always wanting for more and this will show well in a VXR or ST - they're both small-engined, average power lumps with poor fuel economy to boot, all in exchange for a few seconds shaven of your 0-60? Not worth it - especially for 15k.
I'd recommend looking at what you can sell your car for, and how much you can save in the time it takes to sell your car. Add it together, and use that as your budget for a used car. There are thousands of fast cars selling cheap because people get bored of them quickly. They realise the impracticality of a high-revving petrol drinker and move on to something more practical as a daily driver.
My dad has the old Audi A4, which is a 1.9 turbo diesel, and has 115bhp, yet his car does 60mpg, mine does 30mpg. Mine is faster to 20mph, his obliterates mine on 0-60 speeds though. He paid £1000 for his car. I paid (and am still paying) £9000 for mine.
Finance is a really bad idea, and your parents know this (either from experience or just being aware of what goes on) - a lot of dealers will set you up with a bad finance package because you're young and inexperienced.
If you're going to do it anyway, then seriously consider the car that you want. Bear in mind that you will have that car for the full finance period and at the end of it you will have a car worth around £8k. Remember that you will need to make every payment throughout the finance period. Loss of income through a sudden loss of job might make this difficult, and certainly if you're starting Uni at 21 you'll find that you have even less money available to you.
In summary: buy a cheaper car that's just as fast or faster with what you can afford now. You'll thank yourself in the long run.
~Matt