: There are many, many historical examples of empires giving up their lands
: with little to no violence, from India to Hong Kong. Why not choose this
: instead?

Went wrong with Indian independence is that the British Raj, the Britain's governing authority in the Indian Empire, had far too much say in the terms of independence and including the partition, separating Pakistan (and later Bangladesh) from India proper. Racial, religious, and political lines were drawn haphazardly across the former empire with little concern for anyone. Britain just wanted to get out of there as soon as possible even though they really didn't. What was left was over a half-century of conflict with millions of civilian and military casualties, including countless more displaced by conflict and borders.

Hong Kong is a totally different matter. This was one dying empire, Britain, ceding its territory to an emerging world power and empire, the People's Republic of China. While it is nice that the terms included that the HK SAR remains under the "one country, two systems" concept with the PRC, this has honestly been far from the case in practice. Few Hong Kongers I know, even the new ones, abhor Beijing's authority and influence over their affairs. Little concern was made between the UK and PRC about if Hong Kong should be able to exercise their rights to self-determination under U.N. Resolution 1514.