Hi Julien Prud’homme,
Income tax and Capital Gains Tax are taxable in the UK, using the 'arising basis'.
This means that you are taxable on your world-wide income in the UK, in the tax year that it arises.
Article 14 of the Uk / Hong Kong double taxation agreement, advises that if you are resident in the UK, but employed by a Hong Kong employer and undertake that work in the UK, then this income is taxable in the UK, not Hong Kong.
UK/HONG KONG DOUBLE TAXATION AGREEMENT AND PROTOCOL