Just to clear up any confusion:
If you have a provisional licence or full car licence you can do a CBT, this is compulsory basic training this allows you to ride a bike of upto 125cc (if you're aged 17+) as far as I'm aware all routes to a bike licence include this as it's compulsory - also note the T stands for training not test. You must renew your CBT every 2 years, unless you have gone on to pass a motorcycle test. Whilst riding on just a CBT you must display L plates and cannot carry pillion passengers*.
Your options are now:
A1 (light or 125cc bike test) - this is essentially a stupid option but once passed you can tear up your L plates, carry passengers and ride that 125 forever more.
A - This is the standard bike test, once passed you can ride ANY motorcycle you want...however for the first two years the bike must be restricted to a power output of no more than 33bhp. (Or you can buy a bike that's power doesn't exceed this from the factory i.e a 125cc, 250cc etc.)
DAS - Providing you're over 21 years old you can opt for the direct access scheme - when you pass this test you can jump straight on a full power gsxr 1000 and giggle your tits off.
Before doing any of the motorcycle tests you have to first complete a theory test. If you've done one for a car test in the last 10 years then you'll feel thoroughly ripped off as the questions are pretty much the same bar a few (if you're lucky) noddy 'motorcycle' related ones (i.e "a biker has crashed do you a)check his airways b)stamp on his face to see if he's alive")
Some test centres try to push everyone eligible through DAS as it makes them more money, if you're happy to be restricted to 33bhp for a couple of years then it can be a much more economic option.
*unless you passed your car test pre 1990 and are riding up to a 50cc bike...or something like that.