A CBT certificate is issued once the instructor is satisfied that you are competent on the road.
You start the course with an eye test, reading a number plate from 20m away
Then you do some classroom highway code stuff. I got asked to draw a few different road signs (one of them being toads on road :D). Two of the 4 in my class room got about 2 out of 10, but still passed, as it was all explained and they were happy with it.
Afterwards you do some training round cones etc... braking, balance, clutch slipping etc...
Then you are taken onto the road.
In summary you could probably get away ith very minimal knowledge, though for a safe bet, i'd do a half hour swat up, because they will come in handy at some stage!
Of course some CBTers will be more strict than others..