To be registered as self-employed, you would need to advise HM Revenue and Customs that you intend doing some free-lance work as well as keeping your job. At the end of your first tax year, you will be sent a tax return, on which you will complete details of both your employed income, plus your free-lance earnings and expenses. You must tell them within 3 months of starting a new business, in order to avoid a fine. You will be asked to pay any tax due, and then start to make payments on account for subsequent tax years. If the amounts are small, the tax may be collected through your tax code. You must keep proper records of all your earnings and expenses from day one.
You will be asked to start paying Class 2 National Insurance Contributions of �2.10 a week, although if you expect your annual earnings from free-lance work to be less than �4465, you can ask to be excepted from these contributions. You MUST ask for exception in advance. Depending on your earnings from your job, you may also be able to apply to have payment deferred until after the end of the tax year, in case you already pay the maximum in your employment.
As regards VAT, this will only apply if you are making �61000 a year or more.
You should check your contract of employment to see if you are obliged to notify your employer of this free-lance work. Some employers specify that you must inform them, otherwise you are in breach of contract.