The West Derby hundred was originally west lancashire but as a registration district it was adjacent to Liverpool, Shirland is in Derbyshire, I would imagine he moved around as a police officer during his career, as a police constable i would think it would be from his home county he was recruited and that could have been Derbyshire and if he was already in the force it would be a reason for his transfer to West Derby, but it is also possible the west derby marriage is not him, nellie may not have been Elizabeth, her name may have been something else. helen or ellen or Nora or even nell or nellie. 1926 is also too young for him to marry as a police constable as he would doubt he would have the means of support a wife, ii would think his marriage would be a while later, maybe 1930ish but that's just based on guesswork