What would be suitable to wear to a film premiere?

A fairly superficial question, but something I feel a little lost about..

So I am going to the Cars 2 premiere in London, it’s a daytime showing and I have no idea what to wear. I have my own sense of style but I don’t follow fashion and obviously will not be turning up in some Gucci evening gown or whatever. I want something that suits my figure so I don’t look like too much of an idiot I’m a UK size 12 and fairly busty

I am going with a friend who I would like to impress also :S

Any suggestions or links would be greatly appreciated!