Things I found that helped with saving money in London:
Eat breakfast at home - seems simple but I used to skip it and grab something on the way in.
Lunch - Girlfriend and I cook big meals in the evening and then take some in for Lunch the day after.
Travel - I get off a Zone earlier and walk a bit further, it's healthier and only requires waking up 20 minutes earlier.
Coffee - Ditch the Starbucks and drink Instant - not as nice but sometimes you have to make sacrifices
Smoking - If you smoke, it's an expensive habit - I sucked it up and decided that I'd only smoke once a week, perhaps a Friday or Saturday night alongside drinks, I'd usually split a 20 with a mate. This is easier said than done though.
Drinking - If you're meeting mates for a session, try and see if you can have a few beers at someone's house before, or invite them over (Once again difficult if you're flat sharing). Alternatively, hit the cheaper pubs if you can - If I'm not going to remember the night the day after I see no reason not to hit up a local Spoons . Even consider travelling further afield to cheaper areas.
Uber - Stopped being a lazy bastard and catching Ubers everywhere.