Tadevush

Hi, My situation is following: I have been granted stock options for non-public US-based company while living abroad (in Belarus). I had no connections to the UK at that time. Stock award has been fully vested in 2021 before I moved to the UK. I plan to exercise those options now (being a non-domiciled UK tax resident) and want to understand tax implications: 1) Do I need to pay only CGT in this case or income tax and NIC as well? 2) If I need to pay CGT, how do I calculate FMP for the shares at the time of the vest?