I would also question this:
Remember that in the 40s and 50s any load over 1 kW was quite unusual in a domestic situation.
There were plenty of two-bar (and for that matter three-bar) electric fires in use then, more so than today given that central heating was still comparatively rare in British homes at that time. And how about the ubiquitous electric kettle, normally at least 2kW? (Although to be fair, most were probably connected to the single socket incorporated into the cooker control unit.)
Twin tub washing machines with integral heaters became more popular too. While the chances of a washer and dryer being connected to the same double socket have, I would say, increased immensely in more recent times, I think it would be fair to say that outside of the kitchen & utility areas the loads on 13A sockets around most of the house have, on average, probably decreased.