Greenwich Picture House - great seating, bar and an audience who want to see the film. I tried Beckenham but the last time I went to there was an absolute nightmare - it was dirty and members of the audience were abused randomly by a pair of ladies and their delightful offspring. Within about 15 minutes they had cleared an exclusion zone around them. Seems like they were known to the staff who did nothing. I would never go there again.
You can become a member at the GPH which entitles you to discounts not only for the seats but in the cinema bar, local and national businesses. You can book online, choose your seats and the fee is waived. You would need only to go about ten times in the year to get your membership fee back in the savings you could make. It's a real pleasure to go there. I would recommend it most highly.