careful as you might buy a 125, want to sell after 3 months and upgrade insurance to 600 and they either a) charge you a fortune to cancel the policy on the 125 b) charge you a fortune to change your bike details c) charge you a fortune for the new bike The insurers properly have you over a barrel, there's a few posts on here about it. I'm with the previous post, pass the cbt, do a few lessons on a 500 to keep your experience going and practice MOD1, then switch to 600 for a couple of lessons, then do the tests. 3 months will fly past. prepare on a 500, if you can do it on that, you'll be fine on the 600 as soon as you are 24. Probably save you a few quid for some bits when you buy your first bike