My understanding is that you can do so after 7 years legit residence in HK & having acquired PR status. However, as the PRC does not recognise dual nationality, you will have to renounce your existing nationality & provide documentation to that effect.
If you are currently a British Citizen, you can renounce by filling in the appropriate form (downloadable from UK gov website) & get a letter from the UK Home Office confirming your renunciation once it has been approved.
My understanding is that the UK allows former British Citizens to apply for restoration following renunciation - but only once. So theoretically British Citizens can renounce their citizenship for the purpose of obtaining a foreign citizenship where that is required (eg Chinese), and then apply for restoration afterwards...However whether that would then invalidate your newly obtained Chinese citizenship I am not sure, since under Chinese nationality law any Chinese citizen who voluntarily acquires foreign citizenship automatically loses Chinese citizenship. (With the exception of HK-born Chinese where effective dual nationality is tolerated although not recognised, but this doesn't apply to others.)