Depends what you mean by Great, Britain isn't as powerful on the international stage as it once was but it's a more tolerant country that generally offers its citizens a better quality of life than existed in the Imperial era, and it's still one of the world's top economies in terms of GDP, and a nuclear power
Regarding the decline of the empire, that happened gradually, with some events that sped up the process. Arguably the rot set in during the 1870s when the USA and recently-unified Germany began to compete more effectively with British industry. The years following the first world war saw the creation of the Irish free State and the Commonwealth, both examples of imperial decline from a certain point of view. Then you've got the Second World war, with 1941-1942 marking the fall of Singapore and Hong Kong to the Japanese (proving that Europeans weren't always superior). Britain was left bankrupt and in debt to the world's new superpower at the end of the war, with 1947 (independence for the "jewel in the crown") and 1956 (Suez incident) also being key indicators of imperial decline, although Britain did get the welfare state and nukes during this time. The late 1950s and 1960s saw the relatively peaceful process (Aden and Malaya aside) of decolonisation in Africa and Asia and by the 1970s, the process was all but complete with a few exceptions. Unfortunately, the process of industrial decline which began in 1945 was also accelerated during this time and that was partly why Britain ended up being labelled the sick man of Europe in the 1970s, before Thatcher got in. Whatever people say about the UK today, at least things aren't as bad as they were in the winter of 1978-79.