I have been asked to look at this for you.
There is no problem putting advertising on the sides of a moving
vehicle, whether on the sides of a van or on one of these triangular type
However, if you propose LED screens.
I think you will fall foul of the Vehicle Lighting Regulations
schedule 9, part one, subparagraph seven, which says that side
marker illumination must be Amber except when it is within 1 m of the rear of
the vehicle it can be red or in the case of a trailer it can be white when
viewed from the front.
Ultimately, it is VOSA and the police who prosecute these offences
under the Vehicle Lighting Regulations and, Constructions and Use and if you
want to avoid prosecution, I would run this past them. Do it in writing, and
not on the telephone.
That is the only definitive way of getting an answer. I think it
is likely to be negative.
Can I help further?