Saving in London is impossible!
Not at all! you would be surprised how much we spend with little things: a coffee here, chewing gum there, Poundland stuff, sales items that we will never use, the list is endless.
One thing you can do to save money is to split your salary in two: a certain amount goes to your expenses (rent, light, gas, food, oyster etc...) , and what's left, goes immediatly to your savings account. Only touch that money for an emergency . There are some banks that allow you to easily transfer money from your current account to your savings account, Barclays is one of them.
Another way to save money, which is a bit more radical, is to leave your card at home (unfortunately not applicable for the ones with cellphone payment, those are doomed , may God bless them with willpower).
Withdraw the money you need for the week (be realistic) and leave your credit card at home or somewhere safe. It is difficult at first, but it pays off . Keeping track of your expenses for a week by writing down all the things you bought also helps you to understand where your money is going and cut down unnecessary expenses.
Try one of these and tell us if it works, if not, the hell with that .