There is pleanty on the subject on the web, such as http://www.theaa.com/motoring_advice/child_safety/seatbelts.html
My understanding of it is that children under 3 must be in the correct child seat, but over 3 they don't need any restraint in a vehicle not fitted with belts. If belts are fitted the correct seat/booster must be used until age 12 or 135cm.
To use my Scammell as an example; as a locomotive it is exempted from seatbelt requirements, my son is nearly 2. If I wanted to take him on the road in it he would need to be in a childseat until his 3rd birthday, after that he could be "loose".
In a pre-'67 Gipsy (no rear seats) without belts a childseat is needed until 3. (I can see no reason that this should not be fitted using a ratchet strap, as long as it is secure) Over 3 no restraint is required, but he will be getting one anyway. (required in front seats until 135cm/12years)
So far, when we have been to shows he has gone in the car with his mum. When it comes to it I might well rachet strap a car seat in for him.
There is no way I am going to have a three year old loose in the cab, whatever the law lets me do.