DUDES?!
roll cage is for protecting you in case of a ROLL (GOD FORBID!)
If you are a track racer then fill your boots and get your toe down..and win it!
(racing K10's rule)
(If) its for road use (not track) then IMO it would be more cash & brain efficient to join and practice with your local RoSPA Advanced Drivers and Riders Group.
http://www.roada.org/groups/wales/index.htm
(much cheapness for maximum road skills gained....and they're all over the country)
Your insurers will screw you if you declair a roll cage for road use and if you don't declair it then your insurance will be void when accident investigators see your cage
If I've got the wrong end of the stick then please strike me down!
Just thought of something else:...if the roll cage is for asthetics...then maybe you could get a soft one installed (brainstorming here) that does not have to be declaired as a "roll cage"