Previously.tv boards
The folks who created Television Without Pity have a very active set of TV discussion boards that they set up five years ago. The "Sherlock" forum has a good amount of activity; the thread about "The Final Problem" has 264 postings and nearly 12,000 views:
http://forums.previously.tv/forum/76-sherlock/
There are threads in that forum to discuss BC and MF. Because P.tv is a television-oriented site, it won't be the best place to discuss BC's movies in depth, but it would be fine for keeping people apprised of which ones he's going to be in.
Each TV show's forum has a moderator, and from what I've seen so far (browsing the "Sherlock" and "Young Pope" discussions), the postings are intelligent and polite. I think I'll make this my go-to site for reading others' thoughts on TV shows.