Hi CK Wong,
You do not need to register the business as a company.
If you have registered for Self Assessment, as self- employed, then this is all that is required.
When your tax return is processed, Self Assessment will calculate the amount of class 2 and class 4 national insurance that you need to pay.
These payments will be collected through Self Assessment, along with any tax payable.
You can register for Self Assessment, as self- employed here:
Register for Self Assessment
Provided you have not already registered for Self Assessment.
If you already have a 10 digit UTR for Self Assessment, please contact our webchat helpline here:
Self Assessment: general enquiries