The A2 licence is designed for younger riders as there first step into large capacity machines. (19 to 24). Limiting them to 47 BHP / with a power to weight ration of less than 0.27 per Kg or effectively most 500cc or some larger capacity bikes which can be restricted from under 90BHP to 47BHP. If it is over that power to start then there is no way you can restrict it.
Essentially it is exactly the same test as the A (unrestricted licence), and many schools use the same bikes as the unrestricted with the addition of a restrictor kit.
So you can buy a larger bike, and then when you upgrade your licence remove the restrictor.
So why not wait until your 24 and can get the full A licence. Well it is always an option but girlfriends, jobs, mortgages tend to put things off, so you don’t get the bike of your dreams.
If you pass the A2 licence you can take an upgrade test after 2 years and a day, without retaking your theory test. So you could have your R1 or Harley at 21 if you passed at 19.