This article suggests that high earners (over £100,000) should file a self-assessment tax return:
Income over £100,000 Your Personal Allowance goes down by £1 for every £2 that your adjusted net income is above £100,000. This means your allowance is zero if your income is £125,140 or above.
You’ll also need to do a Self Assessment tax return.
If you do not usually send a tax return, you need to register by 5 October following the tax year you had the income.
But I don't understand where this applies. Is this only for self-employed, or would it apply for employed, receiving salary through PAYE?