I suggest you go and have a look in your dinky UK mains circuit-breaker box, where "up" is invariably "on".
So the real question becomes why the UK has the highly confusing situation where circuit breakers or on in the up position but switches are on in the down position?
Its because they use a lot of toggle switches it is a saftey thing. If the switch spring breaks the switch falls down and disconnects the light or whatever its connected to. I expect the H & S directive is on the way to make us change all ours any day now!
Has it come to this? The "Special Relationship" reduced to bickering about the size of plugs? Actually...it's a Sunday night, I quite like the randomness of this argument so far, carry on...