Here's one you won't find in any BR publication
If a Signalman in a proper box where he could see the trains and they could see him received information that a train was approaching with a strange headcode - or he saw a strange headcode before the train reached him - he would put himself where the Driver could see him and, once the Driver had tooted recognition he would slap the top of his head (very gently of course). Most drivers would recognise this , toot and at least prepare to check their headcode. Any that didn't understand would stop or get onto an SPT (Signal Post Telephone).
In the rare event of something showing a red and white on the front staff would get the Driver's attention and point side-to-side. This would usually get the headlights corrected.
Red and white is the combination used (where available) for shunting. White is the normal front end while red on the front is an emergency stop signal for all traffic in the opposite direction. People did not make that mistake!
When a tail light was wrong on an MU in any way the Signalman would turn sideways and slap his backside (gently - not Basil Fawlty style). This would result in the driver getting the guard to sort out the tail light.
A wrong or missing tail light on loco hauled would result in the train getting pulled up at the next box. This was because MU stock could be dealt with inside the train on most occasions while loco hauled could not.
There is a bell code for "Train Past with incorrect headcode" and another for "Train Past without tail lamp" and I can't recall either of them. In fact "Train Past without tail lamp" has two codes. One to the Box In Advance and one to the Box In Rear - and I could never recall which code was which when I had to use them - something like 2-6 and 6-2... anyone got a list to hand?
No train should ever have any head or tail lights carried or illuminated mid train. This would be a "Stop and Examine" if the train could not be readily / safely stopped or, when safe to do so, a case of putting back on the train to have a chat with the crew.
None of this explains the OP!