Avoid supermarket meal deals (except as a rare treat if you must) your brain will be tricked into thinking 'it's only £3' but they will take a huge bite out of your budget if you eat them every day.
same goes for all the costa coffee places that have *helpfully* been installed on uni campuses.
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Supermarket reductions - double edged.
you can score a useful upgrade on what you were going in there to buy anyway e.g. couple of close dated danish pastries instead of home brand digestives... but once you're in there you'll be tempted to reward yourself for getting a bargain by overspending on something else.