@Aleksandra240 Only a tax adviser versed in Croatian tax law will be able to give a definitive answer but USUALLY rental income is taxed firstly in the country where the property is and secondly in your tax residence country. Now if there is a dual tax agreement between Croatia and the UK then any tax paid in Croatia will be credited against tax due in the UK. There are complications due to the differing tax year ends (as I assume Croatia doesn't work to our daft April 5th year end).
I would advise trying to find a firm of accountants versed in tax laws in both countries or you will likely get in a mess.