Hasan Sarigoz

Hi, I am moving abroad on the 4th October 2025, and will be working in for a company in Belgium with no view to coming back to the UK. My question is how do I report this to the HMRC? I would like to avoid double taxation, and also get tax back that has been paid more due to my Tax code. (Reading online on the HMRC website it is very unclear on when and how to do it, it only mentions the SA109 however the document available is for previous ta year 2023-2024) Also can I apply for National Insurance payment? Thanks