You can enter the UK on ‘Leave Outside the Rules’ until 23:59 on 19 July 2021.
From 20 July 2021, if you want to come to the UK, you should apply for the BNO visa and wait for a decision before you travel.
If you don’t, you might not be allowed to enter the UK.
https://www.gov.uk/guidance/british-nationals-overseas-in-hong-kong