Just emphasising Louisa's point, if you can go on a Tuesday or Wednesday, and there are at least two of you then using Meerkat Movies is a good deal as it enables you to get one ticket for free.
I'm not sure how it works if the tickets are different prices - I suspect the cheaper one would be free. Also you cannot book ahead with Meerkat, which makes it less useful for popular movies at peak times. While Picturehouse and Odeon do accept Meerkat, Peckham Plex don't but given their prices I'm not surprised.
MoneySavingExpert has a page on how to get Meerkat Movies - I got mine by taking out a 1 day UK travel policy that cost about £1.50. So long as you go through the Meerkat site then that gives you membership for 1 year.