"The 33 bhp category disappears in the EU from January as more age related restrictions come in, but there will now be a 47 bhp intermediate licence category."
The Harley 883cc Sportster and the 400cc Ninja have about 45 peak horsepower- maybe the UK rule makers looked at these 2 bikes.
My Honda 600cc cruiser has about 33 peak horsepower with a custom exhaust and rejetting, giving it about the same top speed as a 250cc Ninja, but it is slower in corners because it weighs more and scrapes the footpegs sooner, with its lower seat and frame.
"Until he reached thirty, Randy felt bad about the fact that he was not socially deft. Now he doesn't give a damn. Pretty soon he'll probably start being proud of it." from Neal Stephenson's Cryptonomicon novel (1999).