Les - I don't really know the difference in the motor itself. It depends on the design. I would think that both the field coil AND the armature are entirely different, at least in terms of the windings. Likely, they both have windings made of smaller wire and more turns for 240v - I think. Because higher voltage needs less wire thickness, and lower voltage, more thickness. So like a 12v car blower motor has really thick windings, and only the armature has windings, as the field is a permanent magnet (cuz it's DC), and that makes the total number of turns in the motor lower. So the reverse should also be true. More turns = more length = more resistance to handle higher voltage. Smaller wire also adds resistance.
I'm no expert on the mathematical part of electricity. But I'm looking at ohm's law, and it looks like if you have a 7A 120v vacuum (840w), its motor would have a ~17ohm resistance. V/A=R. In order to have the exact same wattage in the UK, which would be 3.5A on 240v, that would be 240v/3.5a = 68.5ohms. You'd need quadruple the resistance in the motor. However, this is a mathematical ideal, because a motor usually has a low resistance at rest and it becomes higher when it's being run, among other factors.
