£3 per person to see a film (including 3D) at the O2 Cineworld on Mondays.
You have to be a Sky customer - but doesn't have to be Sky TV or have the movies package (e.g. I only use Sky for telephone and broadband, but can use this deal).
N.B. If you do have the Sky Movies package, you get free popcorn as well!
From their site:
Every Monday at The O2 Cineworld, you can buy up to two tickets for just £3 each to see any movie you like, from the latest Hollywood blockbusters, to jaw-dropping 3D releases and everything inbetween.
There are two ways to buy your £3 movie tickets. Either print a voucher so you can buy your tickets when you arrive at The O2, or book in advance below. Your voucher or booking will be valid for the following Monday.