See: https://www.gov.uk/ride-motorcycle-moped/new-rules-from-january-2013

Licence types:

A1 - Minimum age 17. Small motorcycles up to 11kw, 125cc. CBT, theory test and practical tests. This licence is restricted to small motorcycles up to 11kw, 125cc.

A2 - Minimum age 19. Medium motorcycles up to 35kw and a minimum 395cc.

DIRECT ACCESS - CBT, theory test and practical tests.

PROGRESSIVE ACCESS - 2 years experience on A1 motorcycle and further practical tests. This licence is restricted to any motorcycle not exceeding 35kw.

A - Minimum age 24 (DIRECT ACCESS) or 21 (PROGRESSIVE ACCESS).
DIRECT ACCESS (MINIMUM 595CC MOTORCYCLE WITH AT LEAST 40KW/54BHP ENGINE POWER) - CBT, theory test and practical tests.
PROGRESSIVE ACCESS - Held an A2 licence for at least 2 years then further practical tests on a big motorcycle (minimum 595cc 40kw 54bph). This is an unrestricted licence and allows the candidate to ride any size motorcycle.

Accelerated access
Riders who pass an A2 licence at 23 and wish to avoid a 2 year restriction to a motorcycle not exceeding 35kw may take Direct Access on a big bike (minimum 595cc 40kw 54bph) at age 24.