You have stated you have an apartment in Liverpool that has been rented for some time but you wish to deaclare rental income from now.
You should be mindful that no disclosure of rental income may result in penalty/fine and interest on tax due not paid. It is in your interest to prepare property income accounts from day one and see what the net profit was each year you had rental income.
You would be able to claim your personal allowance against it being a UK citizen and the tax may not be high.
I hope this is helpful and answers your question.
If you have any other questions, please ask me before you rate my service – I’ll be happy to respond.