Private Residential Relief flat

Hello, I bought a flat in 2017 in Birmingham and lived there until I moved to London and bought a flat in 2020. I have lived here in London since and rented the Birmingham flat out in the meantime. I will be looking to sell my current London flat and hopefully buy a house. The Birmingham flat has been rented out since I moved to London so I think I will qualify for the Private Residential Relief but I am not completely clear on this. I sent a letter to HMRC on completion of the London flat purchase nominating it as my main residence but it was not acknowledged. Does it matter if I chase it up or not?

Hi,
You may have to pay Capital Gains Tax if you make a profit when you sell, or dispose of, property that is not your home. You can work out if you need to pay Captial Gains Tax when you sell, or dispose of your home and find out if you are eligible for Private Residence Relief using the following guidance:
Tax when you sell your home
Thank you.