If you start working for yourself, as self-employed, you will be responsible for paying your tax and National Insurance Contributions (NICs), so you will need to register as self-employed and fill in a Self Assessment tax return.
How to register as self-employed with HMRC?
1
Register for an online account with gov.uk.
2
Wait for your details, which will arrive by post.
3
Complete your registration using your Government Gateway details, as well as information about your business, like trading name and contact details.
Once registered as self-employed you'll have numerous obligations and responsibilities, like completing an annual Self Assessment tax return.