You’ll need a good bit more than the mortgage to cover rent.
You’ll need to cover all the safety requirements of landlords like gas certificate, PAT testing, Legionnaires testing and an increased house insurance premium.
Plus income tax on the rent of 20% minimum or more depending on your level of income.
Some Councils eg Nottingham also charge a fee to landlords, I think £700, to inspect and ensure rented properties meet a certain standard. This is on privately rented not council.
All legal landlord responsibilities will have to be met before the rental qualifies for Housing Benefit.
Also, bear in mind that if you need care in the future , the rental property will be classed as a disposable asset.