I did this, albeit in slightly different circumstances. I had no choice but to rent my flat out but would caution others against it until they’ve fully run the numbers. This includes:
Additional SDLT payable on your house purchase (refundable if you sell your flat within three years).
CGT on any increase in flat property value when you eventually sell it (pro rated to the years you rented it out).
Income tax on profit from rental income (deductible expenses include management/agent fees, service charge and ground rent, repairs and 20% of mortgage interest).
Agent fees, if including management can be 10-20%, payable up front.
Tenant onboarding costs (checks, tenancy agreement, deposit protection, inventory) can be £500-£1000 easily. And are payable each time you get new tenants.
As pp said, any minor repairs/inconveniences need sorting asap. Allow 5% of income for repairs.
Landlord insurance - it’s a must, around £200 pa.
If you’re leasehold, you’ll likely need consent from the freeholder to rent it out.
You’ll need consent to let from your existing lender, or switch to a BTL mortgage.
The timing of my tenancy agreement and service charge means I have a 2-3 months where both the 6 month service charge and estate agent fees are due up front. These months I still have to pay my mortgage but all rental income goes on paying for these expenses. Make sure you have a pot of savings you can dip into to cover you for times like these.
And get used to the fact that tenants might not treat your flat with the same care as you do. View it as an investment, not your home.
There are definitely people who make this work, but it’s not for me.