We'd just had the Korean War, Britain alone wouldn't have won a rerun, the West might. Support for doing it again, without hostility from the other side first is just about unthinkable. Britain might have been able to achieve better terms on Kowloon and New Territories a hundred years previously during Pax Britannica, but those territories were only there as uninhabited military buffer zone for the transhipment port, naval base and coaling station. So when Britain could, no one thought it important, there was no one there.
Development and them mattering came much, much later. Only really after the Chinese revolution started to provide an endless stream of well educated workers and families with a dislike of Communism. Now the UK started moves to democratise - there was getting to be a population that cared, and which fit with the policy of majority rule before independence, in the post war decolonisation era.