I would like to add that there are more than 16k HK students studying in the UK, and that's only counting students doing higher education (ie. in university). There are loads of HK students in the secondary schools as well so students and related traffic is huge, on top of strong business ties between HK and London and tourists.

There was a period of time you can find more than 10 flights between HK and London every day and all of them are either full or very close to full during peak seasons