Why has Britain stagnated for the past 15 years?

A stagnation that does not seem to end            

Once the world's largest trading nation, Britain stood as an economic titan in 1913, with London accounting for half of global capital investment. Back then, the British Empire covered a quarter of the world's land surface, and the pound sterling was a symbol of financial might. Fast forward to today, and the picture looks starkly different. The pound has devalued significantly, and the UK's importance on the global stage has waned. What is the risk of Britain’s economic stagnation continuing in the current decade, and how did it transition from being an economic powerhouse to this state over the past 15 years?