City of London的 URL :按此
Flowchart - can I register to vote(PDF) URL: 按此
Although it is not a new story, this is the first time I find a page (there is a referendum on mayor in Coventry) specifically indicating that Hong Kong citizen with BNO, BDTC or BOC passport can vote. (source: Coventry City Council)
Any previous resident of Hong Kong who holds a British Dependent Territories, British Nationals (Overseas), or British Overseas passport meets the nationality criteria for all elections in the UK.
Any previous resident of Hong Kong who only has a Chinese Special Administrative Region passport is Chinese and may not register.