Hi Alex.C,
Article 14 of the UK / Hong Kong double taxation agreement, advises that individuals, while residents in the UK, who are employed by a Hong Kong based employer, are taxable on the income in the UK on their foreign employment income.
There are futher conditions that can result in the income also being taxed in Hong Kong. Please take a look at
UK/HONG KONG DOUBLE TAXATION AGREEMENT AND PROTOCOL
The foreign income should be declared on SA102 and in SA106 pages of a self assessment tax return. The UK uses the 'arising basis' on world-wide income, by default for taxation.
Anyone who is resident in the UK, but not domiciled in the UK, can elect to use the remittance basis.
This means they declare their overseas income, not remitted (brought in) to the UK, so that it is not taxed in the UK.
Have a look at the guidance on the remittance basis at section 9 of RDR1, as it is not always the best option
Residence, domicile and the remittance basis: RDR1
Thank you