Just got an email from Odeon, join Limitless between 24th Nov - 28th Nov 2016 and get a £30 Odeon gift card. Limitless memberships are £17.99 per month, or £19.99 per month with Central London. Use Promo code BLACK30 when joining. If you pay up front for a year you get 5% discount (£205 or £228 inc London), which would potentially make the membership as little as £14.58 per month (£16.50 inc London) if you figure you would have spent £30 anyway on food/drink/upgrades throughout the year.
How good value the limitless membership is to you depends on where you live and how often you go to the cinema, I'm currently a member because I go once a week (or more) on average. I wish this offer would have been on a couple of months ago when I joined!