(at around 1h 29 min) Orders are given to fix bayonets, and all the soldiers comply. However, when they go over the top moments later, none of their rifles have bayonets fixed.
Tunnel warfare was used first by the Germans. It involved far more people than the film suggests.
Several times Bill salutes the commanding officer without a hat or helmet on. The British armed forces must be wearing a hat in order to salute as does the senior officer. Without headgear soldiers just do a smart stiffening up of the body.