From my own experience, "my father was 20 years older than mother" can mean only that he was several years older, the exact difference being resolved only when birth certificates were obtained.
John Alfred who married in 1915 was probably born in 1875 or later - based on the wife being 21 at marriage and he 20 years older.
Civil registration commenced in 1874 so one would expect his birth to be registered. There is one possibility in my index but the father was Edward. The western region births have not yet been indexed, so there is still hope of him showing up.