@icarus I like the looks of Roy Kinnear and Daniel Mays.  

I'm for David Krumholtz or Hugh Bonneville.

One of the problems i've been finding is that it's difficult to find someone who looks plausibly enough like an older version of David McKenna, but who also looks enough like my own mental image of the Professor.

Obviously its hard to predict how children will grow up to look, and in many cases adults can look nothing like their younger selves (just look at the actor Jerry O'Connell as a child in 'Stand By Me', versus anything with him as an adult) however in a film I think you need to make the visual connection fairly obvious for the audience, therefore I don't think its entirely unreasonable to look for broader-framed actors for adult Digory.

However, the other problem i find then is that most of the actors of this sort of build either tend to be type-cast as burly 'tough-guy' sorts, or comedy-relief characters. Neither of which feel much like the Professor to me. (accepting of course that all of these actors are more than capable of playing against type).

Anyway, here's another handful of actors that I thought of, who i think work well in terms of an appearance match, but potentially veer too far into the comedic:

Jeremy Swift (65)

John Bradley (37)

Nick Frost (53)

Nick Offerman (55)

@icarus on the other hand, knowing that our young Digs has had health issues and is in early adolescence, we can expect that he will grow taller and slimmer in the next few years. A stocky older Prof Kirke isn't really needed, especially if he's in the later 80s or 90s (or 21st C).  [Logos Theatre's CEO Noah Stratton is playing him onstage in LWW currently, and his tall slim look is fine].

@coracle like I said though, having to rely on an off-screen but in-universe explanation for the character's visual change in the intervening period seems sub-optimal in a visual medium like film. Much better I think to visually convey the connection directly, especially if including Older Digory at the end of this film 

The older actor doesn't necessarily need to hit all of the same physical and facial characteristics as McKenna, but I think they need to have enough in common to sell it. 

Keen to hear other people's suggestions though.

I say Hugh Boneville

Hugh Bonneville is, oddly enough, about to start work on a stage play run of Shadowlands in London, where he'll be playing CS Lewis. Therefore he definitely gets meta bonus points. He is also just about within my bracket of looking enough like David McKenna to feel plausible (more so on David's headshots than the on-set photos).

Weirdly,, my previous long term favourite for the Professor was Matthew Goode, who then also ended up playing CS Lewis in Freud's Last Session. However Matthew Goode is well outside my bracket of looking anything like David McKenna, therefore I've sadly had to rule him out.

So I think I've found the best actor so far who looks the most like a grown-up David McKenna to me 

Brendan Coyle (62) from Downtown Abbey.

Whether he would make a good Professor, I don't know (never watched Downton Abbey) but it definitely wouldn't be hard to sell me on him as an older Digory, visually speaking. 

McKenna for comparison:

@icarus I've watched it devotedly, and I agree that he'd be lovely as the Prof.  He plays a very likeable and brave character in Downton, and there's a huge list of other works he's appeared in.  
Even the eyes and nose are right!

I think they chose very good actors to play the Professor in the BBC Narnia and in the Walden films. I don’t know about who will play him in Greta Gerwig’s movies. I hope they will get an elderly man similar to as he is portrayed in the books. But since they are making drastic changes in adaptation I am not sure what they will do.  The professor could very well ask “What do they teach them in these schools?” after finding out about Greta Gerwig’s changes from the book since it doesn’t appear like she will be very faithful to the original story.

Posted by: @narnian78

 But since they are making drastic changes in adaptation I am not sure what they will do.

Well, we shall just have to wait and see whether all the of the changes are drastic or not. We can always hope for the best in the meantime.

As to @icarus' find, I agree that Brendan Coyle does fit the bill for me as to having a good resemblance to the younger actor.  

@glimglum 

I am not so optimistic about what Greta Gerwig will do. She might try to make the old professor look like he came from the 1950’s.  I hope not.  She doesn’t seem to like the time periods of the books.  I still have not decided whether I will watch the film. I won’t like the changes. Greta is probably more interested in selling tickets than having an adaptation that is fairly close to the books.

LOVE Brendan Coyle in both Downtown Abby and North and South. He looks great to be an older David McKenna, BUT I have my doubts about his vibe for the professor. From what I've seen of him, he does soft-spoken but intense, deep, gentle, but a little monotone. I feel the Professor is a bit more lively, witty, and passionate. Almost jolly, quirky? I actually could see Jeremy Swift doing a good modern professor, if LWW will in fact be close to present-day. I looked up David Krumholtz, I wouldn't have picked him if I hadn't seen the photo @icarus posted. He looks really great in a suit/academic attire. I could see him being a good professor.

@anfinwen 

LOVE Brendan Coyle in both Downtown Abby and North and South. He looks great to be an older David McKenna, BUT I have my doubts about his vibe for the professor.

Well one would think that an established actor such as Mr. Coyle would be able to "rise to the occasion" what with his years of acting experience and knowledge gained. It could be a good opportunity and challenge to increase his acting range by playing Professor Kirke. (My two cents.) 😀