If you have net rental income on which you’ll have tax to pay, (rent minus expenses) then you must register for a Self- Assessment tax return and keep a record of your rental income and ‘allowable’ expenses (you can find out what they are here www.gov.uk/renting-out-a-property/paying-tax).
If the rent is lower than the personal allowance of £12,570 -2022/23 (and you have no other income), you will not pay income tax. But if the gross rental income exceeds £10,000, or the net rental income exceeds £2,500, then HMRC will still require a tax return.
If you make a loss on the rental, it is still a good idea to complete a Tax Return so that you can record the loss, carry it forward, and maybe reduce your tax in a later year.