This is why so many people just got rid of there mogs coz its a mine field 
This is not just to do with mogs, it covers tractors in general, a unimog is classed as a tractor, and this subject covers tractors, it doesnt matter weather you get a grey furgy, A massive fendt, or a unimog, these rules still apply.
Infact if anything mogs are better than tractors as they meet construction and use, so they are able to pass an mot if they had to, and in theory can even drive up and down the motorway, although that again is a grey area. so compared with your average tractor a unimog is a rolls royce. Unimogs also do get more leeway from police and vosa, due to the fact that they dont really understand what it is, dwe got pulled up once and were running on red, and the guy just said to get going because unimogs were that much of a complicated topic he couldnt be bothered with it.
the way we were told by a man who was actually a professional in this subject years ago (unfortunately he died

) as long as the product in the vehicle isnt classed as goods, so your not delivering it to a customer (just taking it back to the yard), then you are perfectly fine.
We ran red for years, and were never done, either cause we were in the right or by chance, i dont know.