I was playing a game where you are a fighter pilot over London and I noticed a lot of time you were shooting down at another plane that was over the city. Would this have killed people in the street?
There's [a thread from a year back] (http://www.reddit.com/r/AskHistorians/comments/211nhd/i_imagine_dogfighting_between_two_planes_would/) with several links to other similar threads; answers aren't particularly conclusive, but shrapnel from flak seems to have been a much greater risk.
I can't recall any mention of casualties from falling bullets in e.g. Bungay's Most Dangerous Enemy or Overy's The Battle of Britain; the New Scientist's collection of questions and answers, Why Don't Penguins' Feet Freeze?, does have a question, "Gung-ho Guns" about weapons being fired in the air as a celebration, and one of the answers is from M. W. Evans who says:
"In my youth, I used to collect brass cartridge cases ejected from aircraft machine guns during the Battle of Britain (...) if the projectile is small, like a .303 bullet, it does nobody much harm when it lands."