The reason they think the rocketship may be a missile is because London in the 60s still had lots of unexploded bombs from the Second World War buried under it – people didn’t notice them coming down, I suppose, they were probably busy with the war or something. Even as late as the 80s the occasional bomb was still turning up in people’s gardens and places like that.

(And I agree with you about Red Dwarf, but Dr Who isn’t really meant to be a comedy…)