Here you go, read this
Category A2 and Category A - Standard Motorcycle Licence
If you are 17 or older but have not turned 21 yet or you are 21 or older but are not interested in getting your standard motorcycle licence via the Direct Access route, then you must do your practical motorcycle test on a motorcycle with:
an engine between 120 cc and 125 cc
a max speed of at least 63 mph (100 kph)
If you pass the practical motorcycle test you will receive a restricted standard motorcycle licence, also known as a category A2 motorcycle licence. This licence allows you to ride any motorcycle with a power output not exceeding 25 kW (33 bhp) and a power weight ratio not exceeding 0.16 kW / kg for two years. You will not need L plates anymore and you can carry a passenger on the pillion and can use the motorways.
After the two year restricted period your category A2 licence becomes a full category A motorcycle licence and you can ride any size motorcycle. Note that it does not matter if the two year restricted period comes to an end and you have not turned 21 yet - you still get your full category A entitlement.
The practical motorcycle test for a full category A standard motorcycle licence is done on a motorcycle with a power output of at least 35 kW (46.6 bhp). You can only take this test via the Accelerated Access option or Direct Access Scheme.
If you got your standard motorcycle licence and are in the two year restricted bike size period (you have a category A2 restricted motorcycle licence) then the accelerated access option becomes available when you are 21 or older. With accelerated access you take your practical motorcycle test again, but on a motorcycle with a power output of at least 35 kW (46.6 bhp). If you pass the practical motorcycle test you get your full category A entitlement and can ride any size bike from then on. If you fail the practical motorcycle test your category A2 licence remains valid.
Direct Access Scheme (DAS)
If you are 21 or older then you can get your full motorcycle licence via the Direct Access Scheme. You get your provisional licence and then do your CBT and motorcycle theory test just like everyone else, but do your practical motorcycle test on a bike with a power output of at least 35 kW (46.6 bhp).
If you pass your practical motorcycle test via the Direct Access Scheme you are issued with a full category A standard motorcycle licence and can ride any size bike. The usual two year restricted motorcycle licence will not come into play