Lots of halls allow you to leave your stuff there over xmas and easter. It's also normal in most cities to be allowed to leave your stuff in your second year house over the summer, and you always can over xmas and easter.
British students will tend to drive their stuff home for the holidays, if they have to vacate their accommodation. If you're an international student, and tenancy dates mean that moving stuff into your second year house isn't practical (or you need to vacate over xmas and easter) you can use a self storage company.
Look for one that will pick your stuff up from you - there's no reasonable way that you'd want to start carting your stuff across the city to a self storage place on an industrial estate.
Of course, never leave anything in self storage that is financially or sentimentally valuable, or which you can't afford to lose for another reason.