England is and has always been a 'mongrel nation' made up of many nationalities and cultures. It didn't do us any harm - we had the largest, richest empire in the history of the world.
The Roman Empire thrived on different nationalities too, and look at what that did for the western world.
Actually your tending to rewrite history. You are no doubt referring to the English Empire of old. Its true England controlled many peoples and nations around the world, mostly stealing their wealth and regulating the inhabitants as inferior people. While not quite as bad as the Spanish, French, Russians, and other Euro empires of the 18'th,19'th, and 20'th centurys you werent much better either. On the plus side Englad did leave a civilized footprint in many of their former colonies, its also true they didnt much like giving them up either. Often useing brutal methods to keep control. The interesting thing about England is they were mainly interested in trade and theft. They werent really religious orientated when they raped and pillaged. As far as England becoming a multicultural, diverse nation herself that mostly happened post WW-ll with more liberalized immigration policies.
In my opinion the Brits are the finest friends we have. They are a just, decent people who also have a lot of guts. But dont try re-writing history about your journey to 2008. Yes you had a rich Empire but it was an Empire where you subjugated foreign peoples and stole their wealth and labor. The entire time backing it with a juggernaught navy.
In the same way the Romans thrived on different nationalities too. Their first order of business was to conquer and enslave them, stealing their wealth as well. Its true they also believed in alliances when it suited their interests and as long as the tribes/nations paid tribute to Rome.
Make no mistake, the British Empire, like the Roman Empire was created by the sword.