I don't think you'll find anything too prohibitive in what they have to say. If you like to make a decent job of anything you build, then I reckon you can get a lot of satisfaction out of building your own trailer. It's also tailormade to your requirements. If you want an example of what's legal, a few years ago, I was looking at a small boat trailer made from, would you believe, laminated marine ply. It was,quite simply, a work of art. Drawbar and axle all in one piece, moulded to meet ground clearance and towball height etc. Indespension units and mini alloy wheels. It was carrying a 600lb dinghy. Admittedly the guy who built it used to work for Fairey Marine.