Actually, the 100 year lease was only for the New Territories, which make up the bulk of Hong Kong's territory, but only about half its population; Hong Kong Is

Actually, the 100 year lease was only for the New Territories, which make up the bulk of Hong Kong's territory, but only about half its population; Hong Kong Island and Kowloon were fully annexed by Britain. However, Britain decided it couldn't sustain Hong Kong without the New Territories, and that's part of why it was returned to China at the same time.