cool your horses Jawskk, you're 15, so many things could change by the time you are old enough to ride a 125 (for example the law) when you are 17 riding on 'L' plates might not even be legal.
Or they may have brough out a new 125 that's caught your eye. Or changed the test again. Oooor even banned bikes all together.
You may change your mind about uni or career path, or have gotten on a bike and decided it's not for you. 2 years is a long time when you are 15, lots can happen.
Believe me, when I was 15 I absolutely had my heart set on a Honda Shadow 125 Custom, it's a huge great cruiser, I was absolutely set on it. By the time I was 17 I'd become a strictly sports bike girl, but as it was a friend had a cruiser going begging so I took it. I rode that thing all of twice before I realised I couldn't handle it, it did not suit my riding style and I needed to get rid of it before it killed me on a corner (Tip: don't try and corner a cruiser like a sports bike, it won't work). I'm now 23, I'm back to strictly sports bikes, although I still haven't passed my test (tried a few times last year) I missed the change in test and oppurtunities before that because of financial problems, bike accidents, moving out and having to get a job to pay the bills. Needless to say, I still don't own a big bike and my choice in bigger bike has changed several times just over the last year.
And in response to your original question- no you can't have a bigger bike and restrict it to 125cc, you can't change the engine capacity like that and the rules are pretty specific on the 17 years old only a 125cc bike thing. Even if it was down to Kw or bhp it would still be ill advised, you'd kill the bike. I mean you wouldn't pass your test and get a Thou and restrict that to 33bhp
Hope this helps. Credit to your enthusiasm, we definitely need more keen young bikers.