Do you have the GRO
https://www.gro.gov.uk/gro/content/certificates/indexes_search.aspIt is free to register and view index. It gives mothers maiden if married also ages at death in the early years they don't show on Freebmd.
Civil registration did not start until Sept 1837
To start off
THOMAS, MOSES mother's maiden NICHOLAS
GRO Reference: 1839 J Quarter in HAVERFORDWEST Volume 26 Page 603
Give it a go..
Birth and death certs can be purchased for £3 instant download, if wanted.
Also death for Benjamin looks to be Dec Q 1871 after census.
THOMAS, BENJAMIN age 83 (bc 1788)
GRO Reference: 1871 D Quarter in SWANSEA Volume 11A Page 408
Elizabeth I cannot find in 1881 few likely deaths in Swansea
Come back if stuck..someone here will help