Seconding Broadchurch, it’s incredible. Big Little Lies too. Poker Face arguably counts, with its Columbo-esque “just one more thing” motif. And The Flight Attendant gave me a lot more respect for Kaley Cuoco (of Big Bang Theory fame), though it’s also got some (darkly) comedic elements. Naturally, do a cursory search for all of these before passing them along: I enjoyed them, but idk what your grandparents are like…

You don't have Broadchurch in there. Also Annika. And then, non-UK Mayor of Easttown

True detective is great. At least season 1

The cozy murder genre is my jam. Here’s some more they might want to check out:

• The Doctor Blake Mysteries
• Endgame
• Shakespeare & Hathaway: Private Investigators
• The Glades
• Castle
• The Sniffer (in Russian, with subtitles)
• Burn Notice
• Inspector Morse
• Perry Mason (The reboot on HBO starring Matthew Rhys)
• Justified

(Bonus recommendation, with a caveat: the new limited series Bodies on Netflix— I’m 5 episodes in, and it’s pretty good!)

Surprised they didn't watch Luther.

Monk maybe of their taste, considering mentalist and poirot.

Young Wallander

Psych

I'm not seeing Foyle's War listed anywhere here, so I'll throw that one in. Murder mysteries in Blitz-era England.

Monk! It's funny. Very well done.

Vera is my absolute favorite British detective show! Also really enjoyed Grandchester, especially if they liked father Brown. Line of duty is pretty good too. Unforgotten is really good as well.

Miss Fisher's Murder Mysteries is Australian rather than British but still quite excellent.

Well it's not British and a bit old and I have no clue where it could be currently watched, but my grandparents and aunt watched Columbo regularely.

https://en.wikipedia.org/wiki/Columbo

Life on Mars, then it's sequel Ashes to Ashes and completely separately a show called Endeavour.

I really love the old Sherlock Holmes show with Jeremy Brett. Probably my favorite portrayal of the great detective

Get them a subscription to BritBox, it's almost all British detective shows.

Criminal Minds