Tom
I did exactly that, i got my CBT and then a Yamaha YBR125* to get used to the whole motorbike thing (as I’d never ridden before).
125 was great fun, 200miles per £8 and i gained a lot of confidence in my abilities to handle the weight, speed etc (compared to a push bike!)
Overall, it took me a couple of years to pass my ‘big bike’ test, but i was in no rush, choosing to feel ‘ready’ to step up to the bigger class. My thought was that i needed to get some good miles under my belt before trying bigger bikes. I’m glad I did.
Take no notice of the idiots who say ‘go get a proper licence, big bike, spend loads, blah blah, do what you feel is right, what you can afford, take your time and be safe while having fun.
* I was recommended to stay away from cheap ‘chinese’ brands of smaller bike as they can be prone to problems, falling apart etc. good advice tbh, i bought the yammy and got my money back once i upgraded.