I was watching peaky blinders and my friend explained that thier haircut was a very popular hair cut at the time. Did it become popular because of the helmet styles during world war I? I know the british had a very shallow helmet and I know that they introduced shaving because of gas masks so I wasn't sure if there was a link between the hair style and the helmets.
Short answer: no; the undercut was a pretty common Edwardian-era (pre-WW1) haircut that was very popular among the UK working class, and would trickle into other social strata and geographic areas post WW1. It was pretty common through the mid-40's.