The non-resident landlord scheme requires the tenant or letting agent to withhold basic-rate tax from any rental payments made to a non-resident landlord. The landlord may still need to file a tax return, and any tax withheld is deducted from the UK tax liability.
The landlord can however apply to HMRC to have the rent paid gross, which is generally granted provided that their UK tax affairs are up to date. The rental income and expenses are then reported on a UK tax return.
This often applies to individuals who leave the UK and let their UK property while living abroad. If they were renting out UK property before moving abroad, and keep doing so, they will still have to apply to have their rent paid gross.
The taxable income is calculated in the same way as for a UK resident.