The UK was a superpower (or very strong great power) until 1956. The Suez Crisis is widely brought up here in the US as an cautionary tale of a superpower being brought down due to too much debt. If it avoids Suez or has its desired outcome it could last longer as a superpower.
Suez wasn't that big a turning point. India was gone, Canada, Australia, etc. were for all intents and purposes fully independent, and the economy was only sluggishly recovering from WWII: rationing had ended just 2 years ago. Britain had been trying to punch above its weight for some time - the Suez crisis simply showed the emperor had no clothes.