It does seem strange that the tax break on the vehicle is for everyone. Couldn't you be required to sign a declaration that the vehicle is for agricultural use and commit to only using it for such purposes, before being offered the tax break? I would think most people would stop short of tax fraud at that point.
HMRC know what work you do - so it should be pretty obvious without needing to declare "i'm throwing hay about"
Also, how prevalent is ANPR / numberplate recognition? Just wire that shit up so if anything over 1tonne arrives within M25 you get a £500 bill sent out.
It does seem strange that the tax break on the vehicle is for everyone. Couldn't you be required to sign a declaration that the vehicle is for agricultural use and commit to only using it for such purposes, before being offered the tax break? I would think most people would stop short of tax fraud at that point.