This could well be an issue with the iPlayer add-on, but I’d like to eliminate the possibility of an OSMC issue, if that’s possible…
Recently, playing something in iPlayer has resulted in interminable buffering. Today it’s gotten so bad the video won’t even start.
YouTube still plays okay, even at bit-rates of 30Mb/s, so I don’t think it’s a network or broadband issue.
Does iPlayer use Widevine / InputStream?
I’m fairly sure it doesn’t use Widevine, because the video is definitely decoded in hardware. I’ve always assumed it uses InputStream, but I’m not sure.
Of course, now that I’ve raised this issue, everything seems to be working normally again this evening, after being screwy for several days. 
I’ll complain some more if it recurs! 
Well, the buffering problem on iPlayer has indeed recurred. It comes and goes a bit, but it has been a problem much of the time for the past week. Also, I’ve installed Kodi and the iPlayer add-on on my Fire TV Stick 4K Max, and on there I am not seeing buffering at the times when I am seeing it on the Vero 4K+; so it is beginning to look like an OSMC issue.