Literally Whatever's on at the Old Vic, 103 The Cut; 844 871 7628
There is a Young Vic, and an Old Vic. You, sir, want the latter, because... well, it should be pretty obvious by now.
Holland Park, Royal Borough of Kensington and Chelsea
It's got the most shaded areas of any London park, which is a bonus in basically any weather scenario when old people are involved. Also, lots of benches!