So, you're employed full time and you want to occasionally do some work on the side to bring in extra money...
According to HMRC, who I just spoke with, you don't have to register as self employed, but you do have to fill out a self assessment tax return and declare your extra income.
See Self Assessment: Guidelines for Individuals completing Tax Returns
If you are a company who occasionally employs someone who has full time employment elsewhere, you either a) have to set up a PAYE scheme for them, in which case you are sorting out the tax, or b) you just pay them, list it as outgoings and they are responsible for sorting out the tax.
