freelancer while employed

Hi all
I am currently working full time and I started a small activity as a freelancer in the evening. At the moment I think I am going to earn few money (let's say 200 pounds per month). My question is:
Do i need to register as a self employed? In that case does it affect my net salary as employed? and what happens if I will earn very few money? Any disadvantages?
Sorry for asking so many questions and thanks in advance to whoever will answer
Andrea