Can't speak for the athletics side, and don't know how things are in the UK, but it has generally been my experience that when girls get snide with other girls on their clothing, it's because of envy.

I think you look great in either one, and when I watch women in any sporting event, I'm watching the sporting event. Really don't care what they are wearing. If you are comfortable in the short one, and if it makes it a bit more fun for you, I'm certainly not going to bag on you for it. If you can throw a telephone pole across a field, I'm gonna take you seriously even if you are wearing Hello Kitty pajamas.

Be yourself and have fun. Forget the rest.