Not "official" official, like guvmint-approved. But there are some industry-wide standards, which a huge bunch of shops/manufacturers agree roughly on, so that is about as close as it gets. M&S definitely do have very rigid internal standards on this one, hence the fuss when they "redefined" a size 14 and so on. Most places are in rough agreement with them, with the notable exceptions of Dorothy Perkins and anyone who charges over £60 for a t-shirt when you'll find that the more you spend the smaller the size you appear to be. I've worn 14s in some designers' clothes (not mine, I hasten to add) when I was an 18 most other places.