Hi alextfchan,
If you have a written contract with your Hong Kong based employer, then you are an employee.
In a self assessment tax return, you would declare this income in the employment section (SA102) and the employment/self employment section of the foreign sections (SA106).
If you do not have an employment contract, but will be working freelance, then you would declare the foreign income in the self employment section (SA103) employment/self employment section of the foreign sections (SA106).
You will need to contact the
National Insurance: general enquiries for advice on paying national insurance.
Thank you.