You’ll need a CBT to get on a 125cc. You’ll need to pass an A2 test to get on a machine with 47hp, or an A test to get you on any machine.
You can only do the A licence if you are over 24, in either case you’ll still need a CBT first (one day course).
Both tests have two elements: off the road maneuvers (Mod 1) and then a road test (Mod 2). If you can ride a bike already, then a couple of hours to train for the Mod 1, and then a couple for the Mod 2 could do it.
If you’ve never ridden before then allow around 12 hours tuition to pass both elements. You’ll also need the theory test, even if you have entitlement for a car already.