In practice, most new construction has 20A circuits, but the "standard" is 15A, and they don't want people to have to know if they have a 20A or 15A circuit. But they also usually serve more than one outlet, which means that they also want to leave a little power left over to run the clock in the microwave, etc., so as not to trip a breaker in the very likely event that it's not on that circuit by itself.

I don't know if that's the case in the UK. I want to say that you do have dedicated circuits per outlet. Something to do with wiring in old construction and the reason for there being a fuse directly in the plugs?