Tattie Bogle said
"I'd have thought his singularly best-known/most popular song in the UK was "Last Thing on my Mind", tho' the Zoo song is also a favourite"

Well, I would suggest a huge percentage of the UK population will encounter "Going to the Zoo" during their primary school years.
Not so for "Last Thing on my Mind".
In folk clubs, of course, "Last Thing..." would be the one Tom Paxton song that you're sure to come across.