Why is this so shocking?
It is all based on clothes sizes, which in both US and UK have increased in recent times. So today's size 12 is far, far bigger than a size 12 would have been in the 70s.
I agree it is a misnomer when the papers started wringing their hands over sizes when it is evident that American sizes are used. So a 12 would be a 16. When everyone bangs on about teh size 0 nonsense they are actually meaning a US 0, which is a UK 4. Which, to be perfectly frank, is not necessarily hideously underweight. I imagine that a petite fine boned person could easily be a size 4 and perfectly healthy.
I am 6 foot and a size 8-10 - I would consider myself thin whatsoever. In reality I am probably an old size 12-14. It's just vanity sizing.
I also agree that a UK 12 is considered plus size, as the normal modelling size is a 6-8.