Here's a bit of apocalyptic fun: simulate dropping an asteroid on the UK. It's fun, and when you get your crater you can see how big it is relative to the Sphinx, or the Eiffel Tower, or the Empire State Building. There's a death-and-destruction list too; tell it how far away from the impact site you are and it will tell you just how dead you'll be.
In general, the answer is: very dead.
If you're like me at all, you'll turn the knobs all the way up and see what happens. Yikes. Too bad the simulator only uses a map ofEngland Wales, because obviously if this were to really happen in England Wales the Doctor would save us.
Edited to add: I had originally said this was in the UK in the first draft of this post, then looked at the map and foolishly thought it was just England (Wales is shaped vaguely like England, and I was in a hurry). Silly. I know the difference between the two (or three: UK, England, and Wales) so my apologies to those in England and other places who would not be wiped out by this simulator.
Tip o' the Whipple shield to Orbiting Frog.