I think this rule had been watered down by the 1970s - certainly there was no easy way to observe signals from a guard's coach, after they stopped fitting them with periscopes. It was always conditional on other duties, and the accident reports I referred to above mention reasons such as being involved with ticket checks, writing up the journal or locating mail/parcels to be offloaded at the next stop. However, in modern times, I think it would be unusual for a conductor to lose "situational awareness" sufficiently to be caught doing the washing up when approaching a station stop, though it's relatively common to be stuck with a passenger/revenue issue or delayed getting to a door control position due to passengers getting in the way.
Railways Archive is currently loading a few accident reports every day, dating from the 1870s. Even in those days, I detect that the Inspecting Officer was sometimes sceptical about the truth behind the reasons guards gave for not noticing adverse signals etc. The reports are often object lessons in how you can imply doubt and criticism without it being transparent.
In defence of the guard delayed by washing up; I was alighting from the TRFKB and the guard was a regular on the service, and generally outgoing and proactive. He may have been caught out by the (then) newly introduced C.D.L.. Perhaps more worrying was M.M.L.'s policy of coffee cups being washed in tepid, possibly not-drinking, water by guards - all those germs and virii (??) for transfer between lips and cups of successive passengers....
Also on the subject of conductor guards; long ago, I sometimes travelled home from work on a Blackpool/Barrow service from Euston. Because of the division of portions, it had a brake coach in the centre of the train, usually a BCK. On one occasion, reclining in the luxury of this coach, our departure from Euston was reluctant with a heavy tug and protesting buckeye springs. A few sparks and the sound of frying eggs suggested the handbrake was hard on. I did walk down the train to find the guard, who reluctantly accompanied me back, made a desultory tug of the brake wheel and said nothing was wrong. He said to another passenger 'he worries too much!'. Approaching Rugby, I walked towards the door by the van; the guard was standing the other side of the swing door and indicated 'we got here o.k.', rather dismissively I thought. As we drew to a halt, they smell of burning grease became stronger, and clouds of smoke blew up the outside of the coach. I had a tight connection to the Northampton train; only had time to notice the cherry red wheels. Looking in the control log the next morning showed considerable delays caused by shunting the two portions around an instantly red-carded BCK - Rugby only had one down platform then, of course. Could have had an even unhappier ending - a coach on a Glasgow express in steam days went for a shorter and slower run in a similar situation before the false flanges caused derailment and casualties. We went through a few facing points, some at 100 mph...
