In June 1972 BEA flight 548 crash landed in a field near Staines shortly after take-off killing all 118 people on board. This was Britain's worst air disaster until a terrorist bomb knocked Pan Am flight 103 out of the sky over Lockerbie, killing 259 people on board, but only 11 on the ground as Lockerbie is a relatively sparsely populated area. In January 2008 BA flight 38 ("Speedbird 38") crashed short of the runway at Heathrow after ice crystals restricted the fuel supply.
It is a personal opinion, I am not a statistician by trade, but aircraft can and do go wrong. It is rare but they crash. If you couple this fact with (currently) around half a million flight movements in and out of Heathrow, it is not a case of "if" a flight crashes into a densely populated area, but "when"
Luckily we have an "idiot savant" in our midst to point out, with no substantiation whatsoever, that I'm wrong... oh, hang on, you're not a savant?