As advised by the UK Government, you should not use the NHS testing service to get a test in order to facilitate your travel to Hong Kong. A private test is to be arranged.
You may consider arranging for a test at a healthcare institution in the UK. Please kindly note that 26 June 2021 onwards, the relevant test must be a polymerase chain reaction (PCR)-based nucleic acid test. Results of tests conducted using other testing platforms (e.g. reverse transcription loop-mediated isothermal amplification, RT-LAMP) cannot be considered to have fulfilled the relevant specified boarding requirement. When looking for testing service on the market, please take care to ascertain with the concerned laboratory or healthcare institution that the test report and the documentary proof to be provided will meet all the requirements as set out in para. 2 above.
Whilst London ETO is not in a position to recommend or endorse any private laboratory or healthcare institution, it is understood that some healthcare institutions on the UK Health Security Agency's list of general population COVID-19 testing providers(https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/1069114/covid-private-testing-providers-general-testing-14-4-22.csv/preview) may be able to provide the testing service that meets the requirements. The list is provided only for general reference to assist Hong Kong residents with their research for providers of the testing service. It is by no means exhaustive, and is subject to change at any time as availability of service by private laboratories and healthcare institutions is market driven. None of the service providers listed should be construed as providers endorsed or recommended by our office.
Thank you very much , that link is very helpful
https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/1069114/covid-private-testing-providers-general-testing-14-4-22.csv/preview