Yes, it is more dangerous, but the distance isn't the real issue here, even at altitude - the brekdown of air is going to be comfortable above 1,000V / mm.

The problem comes if you accidentally touch the live plug terminal - the old style English plugs were notorious for children sliding their fingers between the face of the plug and the socket, which is why the newer style has insulation halfway down the pins. The problem is if you do manage to touch the pins while, say, fumbling in the dark in an aircraft, you're much more likely to survive 110V than 240V.