Working and living in UK, tax residence also in Italy. Been asked about documents for taxes

Hi all, I am from Italy and I've been living and working in UK for some years, working for the same employer (PAYE) and generating relevant P60. I have kept my tax residency as well in Italy, in layman terms every year I tell the Italian HMRC how much I have gained from my work, how much taxes I paid and they calculate how much I should have paid in Italy (and I pay the difference). I don't have any income in Italy a part from my UK salary (to make things clear). However, for a specific year 2021, they challenged me about the taxes I paid in UK and requested further documentation supporting my case, e.g. certificate of residence in UK or certificate 0 tax relief. Is P60 generated from my HR website enough? Is there any certification I should apply to HMRC to get this right? It feels like they are asking for double taxation. Any help is appreciated, Cheers

Hi Alberto Acuto,
You can apply for a certificate of residence here:
How to apply for a certificate of residence to claim tax relief abroad
Where there is no source of income arising outside of the UK, a letter of confirmation of residence will be issued, which should be acceptable to the Italian tax authorities.
Thank you.