The level of consequences is very different
All of the points in pjc50's answer, plus, a huge difference in consequences.
Depending on who you believe, Brexit will produce anything from "sunlit uplands" advantageous to the UK"sunlit uplands" advantageous to the UK, orto a plunge into severe recession - but while such economic changes may have serious effects on many residents, for most UK citizens, at least, the basic tenets of the rule of law should continue.
In Hong Kong, what people are protesting is an abandonment of the due process that they are used to, and a change to a scenario where anybody can be removed to the mainland and be subject to the whims of the Chinese authorities.
That's a much bigger shift, if not for the country/territory then for its people.