Brylane, one of the market leaders, sells clothes up to size 60, and finds that a third of its large-size business is in 28 (ie, a 45-inch waist) and over.
On a single morning at Roman Road market in Bow, you'll pass designer clothes, donuts the size of your head, pirate DVDs, and the stall whose chief product appears to be thermal socks (my personal favorite), before nipping into east London institution G.
My current favourite shop is Monki, which looks like it only makes clothes for willowy Swedish hipsters, but their size L is generous enough to fit up to about a size 20.