. . . . . . trade plates had to be inside the vehicle. . . .
That has NEVER been the case.
Current advice from .GOV.UK . . . . . . . . .
How to display your trade plates
When you display trade plates, you must:
put them on both the front and back of the vehicle, or just the back for motorcycles
make them visible and easily readable from a distance of 20 metres
put them on the outside of the vehicle
not cover the existing number plates, unless it’s a motorcycle