To sell an app (or an in-app purchase) on the iOS App Store as a non-US citizen, a tax ID is required* to fill in a W8 form. That is either an ITIN (Individual Taxpayer Identification Number) or EIN (Employer Identification number).
As an individual, the ITIN would appear the most correct option. The last time I investigated it, it was a slow process that would require notarised copies of supporting documentation.
Getting an EIN is much easier, and despite the name, does not require you to have employees. All you need to do is be in business (which selling in the app store is) and being a sole proprietor (sole trader in the UK) counts.
In theory, you fill in form SS-4 to get one. It’s very straightforward, and even has an explanation of exactly which parts you need to fill in (look for notes for “IF the applicant… Is a foreign person needing an EIN to comply with IRS withholding regulations”.
In practice, it is even easier than that. You can apply for one over the phone. They will basically ask you the same questions as on the form, and then give you the EIN there and then. I called “first thing in the morning” (6am for them, 11am for me) and got an answer immediately (they warn you that you could be waiting an hour during busier times). The whole process took less than 10 minutes. The hardest part was confirming how my address should be written down. The net result of all this is that Tic-tac-toe Collection’s single in-app purchase has been enabled (at least for beta testers).
* Whether it is actually required is unclear. The old Xbox 360 program, Xbox Live Indie Games, allowed developers to sell without a tax ID, but a 30% withholding tax would be deducted from your earnings. I believe the best result would be the same happening here. It’s also possible Apple would just not allow you to process sales at all.