CBT isnt a big deal though and you can ride a 125 on L plates, I cant recommend the DT125 enough, the problem is that they have generally been either neglected or thrashed to death by an assortment of 'yoofs', or a combination of both. I know you are happy with your 50 but keep an eye out all the time because if you could net one cheaply or from a mature owner i just know you would be well chuffed with it.
I have a Suzuki Bandit 600, Yamaha DT175MX and a recently acquired DT125R., been riding for 35 Years