Under common law there is a presumption of paternity within a marriage so that is what allows a married woman to register her child and have her husband entered as the father on the entry without him having to be present to agree ( as an unmarried father would).
If a married woman does name her husband as the father knowing that to be false, or impossible, then she would be committing perjury.
Thanks, Antony, I didn't know that. So if a married woman goes to register the birth of her child, and names a man who is not her husband as the father, what information is entered in the register, if he is not with her?
In 1916 would the index still show her married and maiden names?
If the (unmarried) father wasn't present to sign the entry with her as a joint informant, then the father's details would be left blank, whether she named him or not. Her details, if she was truthful, would show her married name and her maiden name which would be reflected in the index.
Many married women having a child by someone else might take the much easier option of just naming their husband - I've seen a number of such examples for women who had been separated, or even widowed, for years. The chance of detection/prosecution would be minimal and it avoided an awkward and embarrassing conversation with the registrar I suppose.
Regarding attestation/enlistment dates you may want to look at the provisions of the Derby Scheme. Many men enlisted under that but then stayed at home waiting to be called for training (my own grandfather was probably one I believe, although his service record hasn't survived).
https://www.longlongtrail.co.uk/soldiers/a-soldiers-life-1914-1918/enlisting-into-the-army/the-group-scheme-derby-scheme/