Updating post from Reddit.
Hi
As a non resident UK landlord does one have to fill in a NRL form to declare tax ?
I believe so, yes. The NRL is more of a "I promise to do a tax return" and HMRC saying "OK, we believe you".
Without the form, your tenants or agent are liable to retain the taxable portion of rent to ensure HMRC get paid. The best logic for this: you've left the country, who's the next best person connected to the property that they could go after.
I am one. You have to declare a non resident landlord to the HMRC and do a tax self assessment yearly.