Hi Chris Wong,
Please register for self assessment as self employed as normal, as a single UTR is all that is required for multiple trades.
This will ensure that class 2 and class 4 NIC can be applied to your net profits. (
Set up as self-employed (a 'sole trader'): step by step).
As you will have more that one busines running side by side, you will need to keep separate accounts for each business.
When completing your self assessment tax return, you will declare the gross turnover and expenses for each business separately, with each business declared on its own individual SA103 supplementary page. If your combined gross turnover from all your business ventures exceeds £85000, then you may need to register for a VAT account.
Have a look at the guidance at (
When to register for VAT) for more information on VAT.
Thank you.