Thanks for all of the advice - and yip, the single track/passing places road was definitely a "There's No way I'm reversing, pal, sorry" experience. . .
As it turns out, I spoke to a traffic cop the other day ran it past him 9excuse the pun).
His position was that as long as it was clear and easily readable (and lit), he'd be happy.
As to mounting on A frame, he mentioned that some new Luton vans have the reg plate high above the rear door, so actual height shouldn't be a problem...
Suspect i should've got his number just in case I need to quote him if I end up on the hard shoulder behind a police Range Rover