Paul Darrow circa 1970? He was 29 in 1970, and I "see" Holland as older than that. i.e. about the same age as Paul Darrow when he was playing Avon ten years later...

I picked up from the books that Holland is British, but then I came to the books because I'd been told that I should go read about this Avon avatar, and that's influenced my view of Holland right from the start.

Irritatingly, I can think of a couple of British actors who might suit, but I've completely blanked on their names. Can see and hear them, but not remember who they are. Unfortunately most of the ones I'd like and whose names I *can* remember are probably too old by now.