That's not at all true. There are many examples, but no need to look farther afield than Ireland, which has not yet recovered from the deliberate genocide of the Irish famine.
The island of Ireland had not yet reached the population it held pre-famine, and if Ireland had grown at the rate of its neighbours, it would now being closing in on 25 million+ instead of the ~7 million across the island.
> The ones who are thriving today owe a huge debt of gratitude, and the languishing ones are that way because they decided not to continue the British tradition.
Britain owes a debt to its colonies. The colonies owe nothing to it, except contempt.
> The 20th century decline of the British empire is one of civilization's worst failures, on par with the decline of the Roman Empire.
In a comment brimming with ignorance, this becomes absurd. The dissolution of the British empire is the great victory of the 20th century. A victory of human rights, decency, and even off the British population, who are disposable to the empire as foreign "subjects" were.