If eating out in London, or anywhere in the UK for that matter, for restaurant chains you should only rarely have to pay full price.
If you register to get emails from places like Pizza Express, Ask Italian, Chimichanga, etc. you'll typically and regularly get voucher codes for 25% discounts or 'buy one main, get another free'.
http://www.hotukdeals.com/ is a good site. You can do searches for just about anything (including accommodation and days out) and get deals.
Travelling by train? Visit this site for two for one deals at places like the London Eye, London Dungeons, Legoland, Harry Potter Tours and St. Pauls as well as dining deals.
http://www.daysoutguide.co.uk/2for1-london
You should rarely be paying full price for stuff here.