I have heard it's only legal if there is internal access from the cab to the living area - don't know how true that is and I've certainly travelled several times in the living of a horse box with no cut through
law states driver must have direct access to passengers (hence cut through) and I believe they must NOT be in forward facing seats (side / rear is OK).
Not dumb at all, a cut through is where you can get from the cab to the living by going through a hole cut in the back of the cab, lorries with no cut through do not have access directly to the livng you would have to get out the cab and go into the living through the grooms door at the side. Does that make sense or have I confused you further?