It's worth bearing in mind that train headlights are less to illuminate the line ahead and more to act as a marker so that people can see the train coming.
Just add another small thing. Class 142s since built (and most still do) only have one headlight. I think its the day setting? Northern have been modifying the fleet recently for both day and night headlights.
In that document, i couldn't instantly see an exemption for LU vehicles.
The answer like many of this kind is to be found back in the 1970s.I'm sure the answer is simple, but why do trains have one headlight on one side and just a marker light on the other? How come it isn't a standard 2 like cars?