Did any UK Mudcatters attend this event at,
I believe, the London Palladium, early 2020?

Mick Fleetwood, the MC and one of the drummers.
No John McVie I don't think.
Christine Perfect McVie performed, however.
Zak Starkey, another of the drummers
Pete Townshend guest appearance
Noel Gallagher guest appearance
David Gilmour guest appearance
Andy Fairweather Low, guitar?
and who WAS the bass player anyhow?
Steven Tyler, guest appearance

Green Manalishi for the grand finale, in which
'Greeny,' the red Les Paul guitar formerly owned by Peter Green,
was played by its current owner,
Kirk Hammett of Metallica,
who played lead guitar and sang.

Oh, here are some of the others:

Guitars: Jonny Lang; Rick Vito

Billy Gibbons, guesting on at least two Peter Green songs

Bass: Bill Wyman

John Mayall,
Neil Finn ("Man of the World"),
Ricky Peterson

AND
Jeremy Spencer playing Elmore James.