New truck on trade plates, what license required?

Followed a clearly brand new chassis cab yesterday being delivered, I'm guessing for a body fitting. At what point does a lorry get plated. The truck in question was probably under 7.5tn so what would the minimum license requirement be. I assume that even an artic unit is under 7.5tn and in theory could be driven on a car license (with the grandfather rights to 7.5tn) before it is registered and taxed. I remember years ago seeing a bus chassis being driven down the road with nothing more than a box to sit on and a flying helmet for protection!

I thought there might be something about the fifth wheel.

Aren't most artic tractors considerably over 7.5t. I think mine is about 9t full of diesel and me

You will need a C licence category to drive a tractor unit without a trailer. There may be a few exceptions such as the urban tractor units like those Mitsubishi's used in London, but the vast majority of them will be over 7.5t. As mentioned, the 5th wheel stuff is a load of bks. 5th wheel or no 5th wheel, if it's over 7.5t and doesn't come with a trailer attached then you'll need a C licence or above.

We might be missing the nuance in Bob's question. He is asking whether a vehicle which weighs under 7.5t and may not have been plated can be driven on a grandfathered entitlement.

I would guess that it can, if it weighs under the limit and doesn't yet have a 17t/18t plate. Im not sure when this occurs, or whether the vehicle will have a weight plate from manufacture, separate from VOSAs 'plate' which will preclude it. If it does then the vehicle would have to be plated down to 7.5t in order to be driven without a C entitlement.

Its something that changed a whole lifetime ago, along with drag & draws being driven on a Class 2 licence and its completely incorrect now.