It is best to register as self employed as soon as you start working. This is because you will need to be registered as self employed to submit your annual tax return.
The deadline to inform HMRC of your self employment during the 2012-2013 tax year is the 5th October 2013. And you would need to submit your tax return by the 31st January 2014. If you fail to submit your tax return by this deadline you will be liable for monetary penalties.