For registration as a register self employed in the UK:

Visit the official HMRC website

Make sure to sign up for or enter your details into your Government Gateway account.

Ensure you enter your National Insurance number and personal information.

Indicate whether you are self-employed and give a description of what your business involves.

Your Unique Taxpayer Reference (UTR) will be sent to you after registration is confirmed.

Following registration, you must submit a Self Assessment tax return each year and pay both income tax and National Insurance. Make sure you register by 5 October at the end of your first year in business.