Hi MS Cheung,
1. You can get split year treatment when you leave the UK part way through the year and will stay out of the UK for at least a full tax year or come to the UK part way through the year and they lived out of the UK for at least a full tax year before they arrived or
accompany a spouse or civil partner when they leave or return to the UK to work full time abroad.
2. If the UTR still not received you will need to contact HMRC to review:
Self Assessment: general enquiries
Thank you.