Firstly, complete the Self Assessment criteria checker to find out if you are required to complete a Self Assessment tax return form for the years in question:
Check if you need to send a Self Assessment tax return
We cannot advise you on what you should do with your tax return in Hong Kong, however when submitting your UK form you can claim foreign tax credit relief on any employment tax paid in Hong Kong to avoid double taxation if you are deemed a UK resident.
You can see the following link to register for Self Assessment:
Register for Self Assessment