You will need to determine your residency position within the UK by referring to the Statutory Residence tests:
RDR3 Statutory Residence Test
If you are now based in the UK then you would need to pay your own tax and National Insurance.
Whether you register as self-employed or not would depend on your employment status with the Hong Kong company.
If you are working for them as self-employed, then yes you register as self-employed.
If you are still classed as an employee then you can follow the guidance here:
When and how an employee operates PAYE on their employment income