Hi,
The tax treaty allows for Hong Kong to tax your rental income first. HMRC will also have the right to tax your rental income in a Self Assessment tax return, under your worldwide income in the foreign section. You will need to work out your net profit using GBP sterling.
You will also be able to claim for interest charged on the mortgage in the box 'Residential property finance costs' on your online return or box 44 of SA105:
UK Property
Thank you.