They had as strict routine to adhere to so weapons cleaning and personal hygiene would be as important as they are today.
Poor discipline though, apparently. Until 1916 every soldier in the British army had a statutory obligation to have a mustache. King’s Regulations :
The hair of the head will be kept short. The chin and the under lip will be shaved, but not the upper lip
IIRC the football truce was on Christmas 1914, so many individuals in the Sainbury’s ad appear to have scant regards for the King’s Regs. All the more surprising as a WW1 soldier could be executed for suffering from shell shock, so you would expect discipline to be well enforced.
Of course the Sainsbury ad with its healthy looking soldiers and their blow dried hair might not have set out to paint a true picture of life in the trenches, it is after all a supermarket, so a romanticized version might have been deemed more ‘family friendly’.