You dont''t have to register any business as a company unless you are going Limited. Just tell the taxman within three months of starting - (they wont forget you, trust me)
Going Limited is nowadays not the tax advantage as it used to be and over the next three years there will be no tax advantage. The only advantages of going Limited are if you run the risk of losing lots of money, Limited Liability will protect you personally and, if you are sick, you get SSP as you are an employee of the company.
The only other reasons for going limited are marketing ones- do your customers expect you to be Limited ?