What a ridiculous rule. Are traincrew supposed to be superhuman beings who supposedly don't eat?*
I've taken many-a-PNB on platform benches happily chomping away at some food and never had a problem. In fact, it's pretty prevalent that many non-London based crews don't like to use the messroom at Waterloo, so you'll often see staff taking their PNB on the balcony or by the new Sidings shopping centre.
*Although I suppose some passengers do think we are superhuman with some of the stuff we get asked.