Hello @Handlez,
Yes, you can use the Office Supplies or Equipment expenses as the category of your transactions. This is because the items you purchased are the assets you're about to use for your self-employed business.
With this, here's an article you can read to learn more about the categories used for your transactions: About SA103F Categories.
But if you're still in doubt, I'd recommend contacting the HMRC so you can confirm what is the best category you can use for the items you purchased.
Additionally, I've also included this helpful article for the answers to the commonly asked questions while working with your self-employed business: QuickBooks Self-Employed Overview.
Keep me posted in the comments if you have any other questions. I'll be here to help. Stay safe!