I am a PPL and know nothing of commercial flying.
I am however au fait with UK / HK taxation and the double taxation agreement which is in place (I lived there for 9 years and hold
PR). You pay tax first and foremost in the country where you are resident. If you are resident in HK, you have no obligation to pay UK PAYE. If however you are UK resident and earn money in HK, you will pay your HK tax. The double taxation treaty then allows for this tax paid to be deducted from your UK tax bill.
You are not taxed twice. You are simply taxed at the same level as if you earned the money in the UK.
If you want to avoid this, become non-resident from the UK.
The Wombat