Answer:
There were a few reasons.
Britain had just fought a word war against imperial tyranny.
British decolonisation had began long before WW2.
Britain had signed the Atlantic Charter and co-founded the UN, both of which dedicated Britain to the ideal of self-determination.
Britain began constantly electing governments dedicated to decolonisation.
Britain wanted out of empire.
So Britain awarded independence to any colony that would take it on British terms.
Independence movements based on Communism, violent tribalism or white minority rule were all crushed to make way for independence on British terms.
Britain wanted to create successful capitalist democracies with which it could remain long-term friends and allies.
With some obvious exceptions, Britain largely succeeded.
Note: It's worth remembering that, aside from the USA, no colony ever won a war of independence against Britain, with even Ireland resulting in military stalemate and political compromise, with Northern Ireland remaining in the UK and the rest of Ireland only leaving the UK to become a Dominion of the British Empire