Hi
Article 14 of the UK/Croatia tax treaty, advises that salaries, wages and other similar remunerations derived by a resident of the UK, will be taxable in the UK, unless you are in Croatia undertaking that employment.
2015 Croatia/UK double taxation agreement.
You should not be paying tax in Croatia for income earned while you are resident in the UK. You will need to claim a refund of the tax deducted by the Croatian tax authorities.
You will need to contact them to advise how you can do this. You may need to apply for a certificate of resident and submit this to the Croatian tax authorities, to claim a repayment of tax.
As you are taxable in the UK, you will need to declare the income on SA102 (employment) and SA106 (foreign) sections of the tax return.
Thank you