Ridiculous or not that's the way it is and I'm sure many get away with....
most of the time
Most get caught out by dedicated squads of police/VOSA/Customs and Excise who set up in random lay-bys and target trailers, mini buses, diesels etc. If you get directed in they give you the works, appropriate licence, dip tanks, trailer brakes, everything.
6 points hurts......especially when your insurance company finds out
It's not a case of not having the right classification, it's a case of driving without a licence for the vehicle and therefore without insurance as far as the law is concerned.