As you are 14, you cannot legally buy premium bonds yourself. Your parents could buy them and hold them for you, mind you.
That said, I'm not a fan of premium bonds. They are a rather weird combination of a savings account and a lottery. Most likely, you'll receive far less than the standard interest rate you'd get from a savings account. Sure, they may pay off, but they probably won't.
What I would suggest, given that you expect to need the money in five years, is simply place it in a savings account. Shop around for the best interest rate you can find. This article lists interest rates, though you'll want to confirm that it is up to date.
There are other investment options. You could invest in a mutual fund which tracks the stock market or the bond market, for example. On average, that'll give you a higher rate of return. But there's more risk, and as you want the money in five years, I'd be uncomfortable recommending that at this time. If you were looking at investing for 25 years, that'd be a no-brainer. But it's a bit risky for 5 years. Your investment may go down, and that's not something I'd have been happy with when I was 14.
There may be some other options specific to the UK which I don't know about. If so, hopefully someone else will chime in.