There are quite a few different questions on (UK) student loans in this forum, and elsewhere, but searches here have not posed the question I am looking the answer for right now. Similarly, searches in the wider web have provided answers which do not look right to me for the answer I am looking for, but perhaps I am misreading the instructions.
To begin, the SLC says for the information they need:
If you're living in the UK
Complete all of your employment details in Section 2. Don’t forget your:
Employer’s PAYE reference number – you’ll find it on your P60 and sometimes on your payslip. It will be three numbers followed by a combination of letters and numbers. Contact your payroll department if you can’t find it.
Payroll or branch number – if you have one, you can get this from your payroll department.
Complete Section 2 with your Unique Tax Payer Reference (UTR) number.
You can find your Unique Tax Payer Reference (UTR) number in the top right hand corner of your Self Assessment tax return. This is a ten digit number provided by HMRC.
And, insofar as HMRC is involved, a director of a limited company is an employee (yes, with both extra rights extra obligations but even so (or not?)), especially if the PAYE system has been set up and regular monthly salaries are being paid through PAYE. However, searches such as this say that everything should be done via the Self-Assessment.
Now, while I agree with the fact that HMRC can only know someone's real income over a year at the end of the annual period when a Self-Assessment needs to be submitted, I am unsure why SLC is completely ignoring that people are probably being paid in part via PAYE and why there's an extra need to report this situation. Can this extra reporting be explained by a logical deficiency of data which would otherwise occur despite HMRC forwarding all relevant information to SLC in any case? After all, a similar situation could arise by dividends from investments, but in such a case SLC does not seem to stipulate a declaration by the recipient of the dividends (though they should still be reported to HMRC via Self-Assessment).
Similarly, if the PAYE threshold was crossed as a director, wouldn't HMRC still be deducting the relevant percentage of all salaries above the Plan 1/2 threshold?
Of course, the final point where the director of the company needs to ensure they have the relevant funds at the end of the year to cover any payments is still immutable (as far as I can see). It's just that it seems ridiculous to have (essentially) double-reporting of what's going on.