You need to join New Era Motorcycle Club
http://www.neweramcc.org/ because they organise the race meetings. You will need the secretary of New Era to sign your ACU license application ...
You need to join the ACU and get a license
http://www.motorcyclinggb.com/index.asp because they LICENSE the race meetings :-)
Both organisations are very friendly, but get rather busy during Jan, Feb and Mar because people ask them for licenses :-)