I'm 22 and a size 6-8 and although this should mean that the high street caters to me perfectly, it doesn't. I don't want cut outs or stupid frills or any bloody polyester and I don't want to look like a teenager trying to get served by dressing 'sexy'. I look at most things in the shops and just think but when would I wear it? I think it is the rise of throwaway fashion and increased competition within the clothing market as they chase profits that has prompted all this hideousness, because retailers expect that you will only want to wear something for one season and want the things you buy to be 'on trend' so they spew out all this horrid crap and wonder why people older than 15 aren't willing to spend their money on it. The shops encourage you to spend spend spend and constantly refresh your wardrobe because they wouldn't make as much money selling classic good quality clothes that people would only have to buy once a decade.
I am willing to pay more for good quality fabrics and classic designs that I can wear in multiple ways, but these things are hard to find unfortunately so I make do with things that aren't quite right and don't make me feel great when I wear them mixed in with the odd perfect piece. One of the best things I own is a navy wool duffle coat like Sali mentions, I've had it for just over a decade, bought it in the Ralph Lauren children's shop (boys section) in Bicester village and it has been worn by my DM, DB and I over the years. If you are petite the age 14 in Ralph Lauren is roomy and the coats are amazing quality and because it's the children's section a wool coat is under £100, cut nicely and classic no stupid unnecessary embellishment, just good quality practical garment that can look relatively smart if needed but equally good on a scruffy day.