Also you stated
Birth Record for Elizabeth Day found (birth quarter 2) birth year given as 1852 mother’s maiden name “Morgan” page 63 volume 11a (doubt about if this is my Elizabeth Day)
The maiden name for the mother of the family Trish found has mmn Cook. She was baptised in 1862 with siblings, the 1861 gives her age 3yrs.
There does not appear to be a registration for Elizabeth, unless she was registered Maria Elizabeth. Registration was not enforced until 1874. Cannot find this Maria Elizabeth in census, but only looked briefly.
Possible - DAY, MARIA ELIZABETH mmn COOK
GRO Reference: 1858 J Quarter in MONMOUTH Volume 11A Page 36
DAY, EDWARD STEPHEN mmn COOK
GRO Reference: 1864 J Quarter in MONMOUTH Volume 11A Page 32
Would keep an open mind until you get the marriage to confirm or rule out.
Also info from GRO, some digital certs can be now obtained at £2.50 for birth & death
https://www.rootschat.com/forum/index.php?topic=875531.msg7473191#msg7473191Added: Thomas Thomas jnr is likely living in Swansea with his family in 1911/1921. I do think the family moved or had links in Swansea after 1891. If you have problems finding him, let me know.
Cas