You have to be a member of a Motorcycle Club that is affiliated to the Auto Cycle Union, the ACU for short.
The ACU is the governing body of motorcycle road racing in England and Wales, including the Channel Isles and the Isle of Man.
The cost of Club Membership varies but is normally a few pounds each year.
You then have to decide which Club(s) you wish to join. You may have to obtain information from several Clubs to help you decide.