Yep totally agree with that, and that it’s a grey line, however, those laws are usually in relation to vintage cars which never had rear belts.
Josie’s defender is a 1998 which, if it was fitted with back seats, most definitely would have had seatbelts. I may be wrong (happy to be corrected) but I don’t think hers had any seats originally and they’ve added in the benches to make it look like an authentic farm vehicle. To me, legal or not, it is madness to travel with small children in a car with NO restraints, especially down fast country roads. It’s certain death if there was a serious accident and I would be so terrified of being responsible, when they could have easily taken another car that has seatbelts and could fit car seats in.