In HK, some people are trying to separate a Chinese city from China. And these people are carrying past colonial powers' flags and asking their support. This is the definition of treason. On anywhere on Earth.
No, it's not treason "anywhere on Earth". For example, it's legal in the UK to campaign for any part of the country to become independent. There was even a referendum on Scottish independence in 2014. Quebec has also had
two independence referenda. None of those referenda would have been possible if it was illegal to campaign for independence.
Also, the Hong Kong protests did not start out about independence. It was the extradition law. But because the Hong Kong authorities and Beijing handled it so badly, some protesters have gone as far as to campaign for independence. But the independence movement is still a minority in Hong Kong.
As for the old HK flag, if some protesters are unhappy with the status quo it's not surprising if they look at the past with rose-tinted glasses.
Final reminder, China signed a treaty with the UK for the return of the whole of Hong Kong (only the New Territories were originally to return in 1997). Rather than divide the city, the UK negotiated a treaty in good faith to give up control but for it to enjoy 50 years of autonomy. It's perfectly reasonable that Hong Kongese would call on the UK to intervene if Beijing is breaking the treaty by undermine said autonomy.