Not getting a CG 125 anymore, then?
Marauder 125 - Good little bike, especially if you're not taller than about 5' 8". My g/f had one and loved it to bits. I had my first go at riding round a car park on it too
Great to learn on, performs well and never heard much complaint about it. The one in the pic looks OK from what I can see. Don't like the colour myself, but it's about what you like.
Parkers may list the price quite low, but I've never seen decent ones going for less than £1200 privately. However, follow the advice of Lady Bella. You can haggle 'em down and ought to at least try.
As always, I'd advise taking an experienced and knowledgeable biker mate along to check it out mechanically. If you haven't one of these, take a copy of Bike Trader - The first few pages (after the ads) have info and advice on what to check when buying a bike.
My personal advice, though is to go for the lessons and full test, if you can. You'll ride so much better from Day 1 and can then get anything you want.
You can't do DAS yet as you're 17, right?
Get a restricted licence before the law changes.
Ideally, I'd suggest getting this bike, riding about for a bit, do the lessons and test on it then either de-restrict it or sell it to buy a bigger one (it shouldn't de-value in that short a space of time).