Hi karencky,
Yes, If you are resident in the UK, while undertaking self employment work in Hong Kong, then your gross profits are taxable in the UK and not Hong Kong.
Overseas income is criteria for completing a Self Asseesment tax return (SA100).
You would need to report the gross profit and expenses on a Self Assessment tax return supplementary page SA103.
Article 14 of the double taxation agreement here:
UK/HONG KONG DOUBLE TAXATION AGREEMENT AND PROTOCOL
Advises that if you are a UK resident, you need to be physically in Hong Kong, undertaking your self employment work, for it to be taxable in Hong Kong.
Where you undertake that work from the UK, then it is taxable in the UK.
Thank you.