Hi Sam, there's a few of us local to you (Nailsea) who race with AWDC, and I know of someone else in Clevedon building and 100" comp safari racer.
If you are looking to buy a cage, or have one fitted, rather then build from scratch I would look at....
http://www.whitbread-offroad.co.uk
http://www.northoffroad.co.uk
http://www.ppcages.com
If you're looking at building then there is a lot of good info in the tech archive as white90 says.