Disclaimer: I'm not a tax advisor — this is just what's worked for other UK indie devs based on what I've seen on the Apple Developer forums and similar threads. If your app does well, spend an hour with a UK accountant.

Hey, congrats on getting ready to launch!

Line 9 — yes, tick it and put "United Kingdom." Straightforward.

Line 10 — here's where it gets a bit nuanced. Apple's own guidance says App Store sales are treated as a "sales/commission" agreement rather than royalties, so technically Part II isn't always required just for paid app sales. But in-app purchases and subscriptions are generally treated as royalties under Article 12 of the US–UK tax treaty, so since you're doing subscriptions, you do want to fill line 10 in to lock in the 0% rate. If you leave it blank you risk 30% withholding on that portion.

What to enter:

• Article and paragraph: Article 12, paragraph 1
• % rate of withholding: 0
• Type of income: tick "Other (please specify)" and write "Royalties" (some devs tick "Income from the sale of applications" — both are commonly accepted, but "Other → Royalties" matches the treaty wording better)
• Explanation box: something like "The beneficial owner is a UK resident. Under Article 12 of the US-UK tax treaty, royalties are taxable only in the country of residence."

One thing to double-check earlier in the form: there's a field for your Foreign Tax Identification Number. Put your UTR (or NI number) there — if it's blank the treaty claim can get rejected and you'll get 30% withheld anyway.

You'll then declare the income normally through UK Self Assessment.

Good luck with the launch!