You don't have to register for VAT until you have a turnover over of 26k (afaik) in any rolling 12 months. You don't need to register a company, you just "trade" under your own name. No registrations necessary. As mr. doran said, you do need to file a return of monies earned in the year. Have a chat with an accountant about your requirements.
A lot of people doing "nixers" outside of their normal job don't declare the money earned and just keep it all, e.g. hairdressers. But I didn't tell you that!
