I mentioned earlier that my illegitimate ancestor's father's name was included when he married in 1842. Were there any rules about that?
The father's, now both parents, information has never been part of the legally required information that has to be given when marrying (in England/Wales) so it is just whatever the bride/groom give. It isn't checked or verified at all. There has never been any rule to stop the father of a child born illegitimately being named, although I expect some vicars may have discouraged it.
Currently it is OK to name a natural, adoptive or step parent.
You can also choose to leave it blank if you want to as Boris Johnson and his wife both did when marrying in 2021 and as far as I know neither were born illegitimately, and both know who their parents are. I use a copy of their marriage certificate as an example of the new A4 format when I give talks on marriage registration. They aren't that unusual either - I had a few similar examples when I was a registrar.