There are quite a few differences in the loads that can be found in American households compared to their British counterparts. The most significant of which is the presence of heating, ventilation and air-conditioning (HVAC) systems in American households. In comparison, UK households are far more likely to be heated by gas, and not require air-conditioning at all. Since the HVAC system often constitutes the largest electrical load in American households, the problem of disaggregating the remaining appliances is clearly simpler for UK households. Furthermore, in my experience, American households contain not only a wider range of loads (pool pumps, etc.) but also contain more duplicate appliances (2 or 3 fridge/freezers, etc.). This again contributes to a harder disaggregation problem for US households.
American and British households also vary in the way that electricity is supplied to the properties. UK households typically receive single-phase (1 electricity input wire) power at 230 volts, while US households receive split-phase (2 electricity input wires) power at 120 volts. The case of split-phase power provides a convenient opportunity to install two current clamps instead of one. Through this small additional installation cost, the complexity of the disaggregation is more than halved, since most appliances are connected to only one of the input cables.
To the best of my knowledge, the capabilities of smart meters in both countries are yet to be finalised. However, the UK provides an interesting extension to the government mandated smart meter rollout, in which households will also receive an in-home display (IHD). IHDs will primarily provide a household's occupants with real-time information, such as their current power demand and cost of electricity. However, since these devices are likely to have access to electricity data at a higher granularity than is transmitted to the energy provider, they provide a convenient platform on which disaggregation can be performed. Furthermore, performing on higher granularity data within each household even circumvents any privacy concerns related to transmitting private data outside of the home. However, IHDs will clearly have limited resources in terms of processing power etc., which raises the interesting field of resource constrained energy disaggregation.