Yo, If youve got a full car licence, all you need is a CBT to be able to ride any 125, Your insurance would be about £400 for a new cbr125, and similar for a DT125 which seems to be what your after. As for the CBT im sure you could arrange to take your own bike, But they always supply them as part of the fee, So you might aswell take advantage of that. CBT's are really easy, You'd have to be blind in one eye and have no feet to fail(they only take about 5 hours). I think a CBT lasts for a few years, But you dont need to worry about that, As you'll get the bug and will booking up for your full bike test in no time