You do not need to register as a business, but you must inform the Inland Revenue within 3 months - the sooner the better really. Here is the form:
http://www.hmrc.gov.uk/forms/cwf1.pdf
Think very carefully though - no sick pay, no paid holidays, no insurance in case of accidents. If you want to borrow money or take on a mortgage you may find that the interest rates will be higher.
If you go ahead with it you will need to keep simple accounts. Records of all monies coming in and all monies going out in relation to your work. You will have to keep receipts.
Put money aside every week to cover your tax bill in the event that you have to pay tax.
Good luck!