I suspect that this is a tip I and lots of other people have already shared but it does save so much that it is worth repeating. Bring your food everywhere !
I pack a lunch every day and it costs me around 70p. There is no way I could buy lunch anywhere for that price. I had to stay late at work yesterday and brought my own picnic tea as well at a cost of 50p. Compared to a Boots meal deal I am saving the cost of at least 4 lunches weekly which adds up to a substantial sum over the month.
I was very tempted by a cup of tea at the station the other day when I was out of water and it was raining. The cost of the tea was £1.60, at the end of a busy week it did not seem much. Then I reminded myself that the cost was more than I was paying to make 1 journey on the Underground. I decided that the tea could wait until I got home !