I just can't help but think that that's not true. That this is the average has been floating around for years I know.
But whilst I do see some people who are size 16 or above, I do not think I see so many as to tip the scales to the extent that 16 becomes the average.
I would say that the average is a UK12.
(I'm not talking about on Mumsnet as the vast majority here are pretty slim and around size 8 average from all the threads that I see either about clothes sizes or posters who mention in passing on a related topic.)