I'll lend a hand to your query about categories in QuickBooks Self-Employed.
Though I'm unable to provide you a specific category to use, I can point you in the right direction. Take a look at this article: About SA103F Categories. It has a list of categories available in the product, as well as the box that it affects.
I suggest you open this article as well: Make Self Assessment easier. It has 5 tips you can follow so you can make this process easier.
Feel free to add a comment below if you need anything else. I'll be sure to get back to you.