I wouldn't say the UK is now, but was the most powerful empire to have existed. What else would you call a nation that 65 nations have claimed independence from? They can't claim independence if they weren't once part of the empire.
At the risk of repeating myself, those 65 nations were comprised of 80% impoverished Indians who struggled with decades of recurring famines. Power was being defined on the European continent through much of the 19th century and 20th century. See the intro to "The Grand Chessboard" where the author expounds on the idea that only Napoleonic France and the US came close to world hegemony, as well as my other replies in this thread.