Depending on how old you are and when you passed your driving test, you may be able to ride up to 125cc with 'L' plates.
You may have to and indeed you should do a CBT (compulsary basic training) first though. This is normally carried out in a playground or car park then a short time spent on the road. There is no formal test. When you have done this, 125cc with 'L' plates is your limit. You cannot use the motorway until you have passed the full motorcycle test (class A on your licence). I think the bike has to be over 50cc for the motorway also.
After that, presuming you are over 21, you can go straight for a 'Direct Access' scheme which will get you a full licence in about 5 days. This costs in the region of �400 and will allow you to ride anything you want. Or you could go part-way and get an A1 licence which will be cheaper, take about 3 days and leave you able to ride up to 33bhp, no 'L' plates.
I would forget about automatics, as you said that you wanted a motorbike, not a scooter.