My lad is 11, but doesn't know (just asked). Agree it's not an easy age - too old for toys, too young for clothes, etc.
If she likes horses (my nearly 8 year old does too), how about a horse notebook/pen/folder/diary, etc to copy Tan's idea - my girl loves writing/drawing in notebooks too or a calendar if they're already in the shops. If she has a mobile phone (most 11 year olds seem to in the UK?) then maybe a horse phone sock or charm (or whatever they're called).
For what it's worth, we have a 10€ limit on pressies for the kids' schoolfriends, a bit more on closer friends - even sticking to budget it is an expensive business.
Good luck.