hi all,
I am not sure of the legality side of the camera systems, afterall they are supposed to be revesing cameras (as apposed to being on all the time in motion) where reversing cameras are fitted they usually only work when in reverse.
there was a longish debate on this very subject a while ago, I don't think there was a definitive answer however,
in theory all one is doing is to substitute the normal view (through the mirror, now obscured by the van) to a screen. and if this screen is in the same place as the mirror (as used by some models) this could be used as a good arguement should billy blue hat give you a pull.
I did look at a few systems last year, some quite cheap, after a incident with a HGV, a motorbike, and a bridge (long story) nearly took out the rear of the van, but in the end decided not to bother as my towing miles per year are quite low, and the legal side of it conserned me greatly.