Hi,
Article 6 of the tax treaty that exists between the UK and Italy allows the country where the property is located, the right to tax income and capital gains arising from the property. The treaty also allows the UK to tax the income and gains as well, but only after Italy.
UK/Italy double taxation convention
As a UK resident, you would be required to register for Self Assessment and declare the gross rental income and expenses incurred in a tax return (SA100) and on supplementary page SA106.
Check how to register for Self Assessment
You can claim a tax credit of up to 100% of the tax paid in Italy against the property, so that you do not have to pay the same tax twice (double taxation).
Thank you.