State Pension - Marriage

Linton

AFAICS from the government website you can only inherit from your spouses SP:

1) some of additional (from S2P and SERPS) or half of extra (from deferring) State Pension, and then only if the pensioner reached SPA before April 2016.
2) half of any pension above the new basic amount arising from previous SERPS/S2P if you were married prior to April 2016.

Or a woman may get extra pension if she paid married woman's stamps.

So in the case of the OP's parents it looks like it inheritance of SP isnt possible.

See https://www.gov.uk/new-state-pension/inheriting-or-increasing-state-pension-from-a-spouse-or-civil-partner