My question is (or was) intended to be simple.
Travelling from (say Lossiemouth) to Marham, at whatever subsonic speed is possible, (and neglecting the time taken to reach 50,000 ft), would it take longer to travel at altitude rather than at 500 ft?
Considering the Earth and the space above it to be a sphere, the suggestion seems to be that flying higher would be further, but the difference relative to the circumference of the Earth suggest that the difference in distance would be negligible.
There is the complication that the speed of sound reduces with altitude due to the decrease in temperature, but there might be legislative limits to maximum speed at 500 ft.
Speed of sound at sea level 760 mph, speed of sound at 50,000 ft 670 mph (or thereabouts).