Considering most people on this platform are based in the UK, I'm not sure if we can be of much help in regards to HKU. I can give some insights on KCL though. I go to LSE so am opposite one of the main KCL campuses and have a friend who does law there. From what my KCL law friend has told me, the teaching quality is very good. Having visited a lot of the big commercial law firms myself, I know they regard KCL highly, although it can be a lot harder to secure a training contract in London as an international student, regardless of the reputation of the university you have attended. Also, Law firms now put much less emphasis on the university you have attended and really focus on the quality of the candidate instead.
From what I've seen and heard, KCL is very international. Most international students tend to hang out with others from their country, especially the international students from parts of Asia as they make up a substantial proportion of the student body. There are also clubs/societies specifically for Asian students and they tend to hold social events such as club nights with those from other London universities every term or so.
London as a city is very fun and it's impossible to run out of things to do. Transport is also very good which makes it easy to get to different parts of the city. London can be expensive though, so most uni students tend to just hang out with each other at their accommodations. Another downside of London is that it is very busy, although I can imagine the same can be said about central HK.
Overall, although I don't know much about HKU so don't know how it compares to KCL, I would recommend choosing KCL if it is financially viable for you. It is a great university with a strong Law department and a good social scene. London is also the perfect place to spend your student years and you may never get the opportunity to live in London again.