BUT, and it's a big but, Britain only survived and flourished because the various component tribes and nationalities lived under a common umbrella. People were very much British first, and the various tribal loyalties came second. In other words, there was an overiding arc that united all these disparate groups. In modern Britain that doesn't seem to be happening - perhaps because, as you say, its' all happened so quickly - and that is going to mean trouble. Multiculturalism is fine, IF people rejoice in the differences. But modern Britain (and the modern USA) seems to be becoming defined by those differences, and that's not a good trend.
Britain and most of the west have forgotten what got them to where they are and have tossed God aside. The result is what we see here.
How is that contraception working out for you? Not very well, apparently.
Even with their fairly common genetic background, and at least reasonably similar cultures, look how much the English, the Scots and the Irish fought each other over the centuries. Throw in a bunch of ill educated, religiously bigoted people of a completely different culture with a history of violent hostility to the West, and what could possibly go wrong?