That's not a valid outcome of an auto-enrolment assessment, you would have to speak to the API developers to understand why that's an option in the API

Does this answer your question? How to get the tax back for over 100k part

Apple have region-specific app stores - so presumably your apps are on the UK app store and therefore generate UK income?

@Brendon Not sure where you got that from but it's simply not true. You can refuse to pass your P45 to your new employer and submit a new starter form (aka a P46) which will temporarily place you on an emergency tax code but by your second payday your employer will receive the data electronically via RTI anyway which will in turn place you on the correct tax code. Your employer legally has to know the information in order to operate PAYE and meet statutory compliance.

fwiw, tax calculations are only done on full pounds anyway - so rounding down is actually the correct way to do it :)