I must admit, I find the 'borough' thing quite an odd argument. It's not a council-provided service, and the last time I looked, transport across borough boundaries did not require identification documents, driving on the other side of the road, or changes of train. Almost everyone in SE23 lives within 20 minutes' walk of another borough, many of two, and the various cinemas that SE23 residents use currently would continue to be most convenient for most even were a cinema to open in, say, Grove Park, thereby ticking the borough box. Likewise, a cinema at the Capitol is probably not going to have the residents of Lee Green jumping for joy.
Yes, this is compensated for by the people from the nearby bits of Southwark, etc, but that just underlines what a red herring the borough argument is.
I remain of the opinion that a cinema in the Capitol would quickly become a white elephant, even if it showed the perfect mix of whatever dismal superhero nonsense passes for mass-market entertainment these days and interesting Iranian coming-of-age dramas about stoats. I may be wrong about that. But I wouldn't risk any of my money finding out.