A recent update to ETS changed it so that UK Plates on trucks would also be the same as trailer number plates. When the update first came out the custom registration I had on World of Trucks worked and did display on the trailer plate. However, for some reason after changing the plate on the WoT website, it no longer displayed on the trailer.
I have tested without the custom registration and the randomised number plate given by ETS on the truck does display the right one on the trailer. I think it might be something to do with my World of Trucks profile but I haven't been able to find out how to resolve this.