damn clothes sizes

Given that the average womans dress size in the UK is a size 16 (USA 14), I was sorely disappointed when I went clothes shopping today and found it difficult to find a skirt I absolutely adored would not fit me.
I am around a UK size 10/12 depending on where I shop, but am slightly disproportionate between my hip/waist measurement.  This makes finding jeans, slim fitting dress or skirts a nightmare to buy.  I do however know what suits me and how to disguise the bits I may not like as much.
I went shopping with an idea in my head of what I wanted to buy, and was overjoyed when I found the perfect skirt in a small Bristol Boutique.  It was a 1950's style swing skirt, black with red roses and was perfect for my iron fist shoes.  In I go to the changing room with a size medium and a large, just in case.
I was horrified to find that not only could I not fasten the large size but the XL I later found was also too small.
OK I had eaten a little earlier on but even so.....
Who on earth are these shops catering for? What sort of message is this giving and what damage is this doing to peoples self image? I know I preach about numbers being insignificant but this is bloody ridiculous.
Rant over.......I went on to a large department store and bought a dress I was comfortable and happy with and in MY size. Sort it out retailers!!