I grew up on the south coast of England in the post-war years. I clearly remember that after a big storm, sections of the beach would be out-of-bounds because a mine had washed up, and the Army was called in to either safely detonate it or render it harmless. It was common enough that it happened several times over a couple of years in the area where I lived, so I imagine overall it happened a lot. As Rainer said, there were a number of other priorities in those years.