I thought, no matter on distance, either a board had to be fitted, or an occupant in towed vehicle had to do the signalling
Towing with rigid bar so towed vehicle will be manned
7 pin trailer electrics are not testable for MOT because there is no set in stone standard of pin configuration
(though many MOT testers fail to read the manual and test anyway 
)
13 pin towing electrics are testable (and can be a fail if not working)
security of the towbar ,mounting area etc is also testable
a canbus relay can be had for £20,simple to wire, so £10 for cable and a 7 pin socket ,£30 total spend
when towing, it is good to show other road users what your doing, like braking,turning, hazzards if you drive like a snail 
Even with a canbus relay, wiring can be a pain - particularly in the case of our vans where the entire rear load area is sealed
I would have thought it would be worth the extra expense of fitting the electrics to maintain the re-sale-ability and resale value? 
Who is going to buy a van with a tow bar that they can't use on the road?
By the time our vans have done upwards of 350k miles, plus the various holes from lights, vents, etc, the resale value is pretty much the square root of FA anyway
Any one able to give a
legal view point please?
(Which was what I asked, as the various other arguments have been discussed before asking here)
