You need to budget so that you don't get into debt. A good method if you aren't good at budgeting is putting the £30 you have in your wallet/purse at the start of each week and saying to yourself 'this is all I have to spend'. This way you can't spend more than you have. Most important of all, do not use a bank card as it makes it so easy to just spend money away.
You don't need money to have a good time during fresher's - (decent) people will judge you upon how you act/speak towards other people and your personality as an individual, not on how much you spend.