Late to see this thread but if you think back to the early days of electrical engineering many switches were of the knife variety mounted on a slate board as often depicted in Frankenstein type movies. Though some mounted the other way did exist the easiest way to make such a switch stay in the off position is to hinge it at the bottom so when pulled down it hangs there ,it isn?t going to close itself against gravity.
One hinged at the top which would need moving up and without additional parts or a wedge to keep it there could easily fall down into the the live position with gravity assisting possibly initiated by vibration in an industrial setting such as a plant stamping out parts. I believe the US mandated that if there were no additional locking mechanisms simple knife switches had to be bottom hinged early in the 1900?s and the convention for up for on and down for off got established. Why the UK went different for simple one way domestic lighting I don?t know but as others may have noted other switches we use such as in consumer units and RCD devices do follow the up is on down is off convention.
GH