As a long-time fan of post-apocalypse scenarios in games, books and TV, massive asteroid strikes have always attracted me. Their unstoppable nature and the amount of damage they can do create the perfect doomsday scenario. The best example of this is Niven & Pournelle’s book Lucifer’s Hammer which is far better than any of the numerous films made on the same subject (Meteor, Deep Impact, Armageddon).
Asteroid’s strikes can be global events that permanently change the Earth, such as the one that killed the dinosaurs. But even a relatively small one can have a huge impact, such as the Tunguska Event (also here). The amount of damage caused relies on numerous factor, ranging from the obvious like the speed and size of the asteroid to the less obvious such as the type of rock it hits and its angle of approach.
Now you can experiment with these different factors by dropping asteroids on Wales using the Impact Calculator. Destroying Wales is a worthwhile activity in its own right but now you do it in a spirit of scientific enquiry and whilst planning your next post-apocalypse RPG.