Largely they have the same power output. While the total power available is actually higher for the US version at 12kW versus 11.1 kW for the UK version. That is because the US version needs to be on a 50 amp 240v circuit versus the max 16 amp circuit for the UK version.
The max output to a single pan/pot typically is going to max out around 3.3-3.7 kW. I don't know the exact reasoning behind that but I assume it might cause issues with cookware if you go much higher than that.