I agree with kurosagi01's suggestion. From that store it's by far the best choice.
I'm just going to throw another option from the same store:
The Acer Ferrari One. This one is a bit more expensive and less performant (the HD3200 IGP is underclocked and the dual-core CPU is only @ 1.2GHz).
However it's a 11,6" ultra-thin laptop. It's a lot smaller and lighter, which could be more suited for a 12 year old girl. The vibrant red colour should also make it more likeable than a black one, for a child. Plus, the battery life is quite decent (around 3.5 - 4 hours) and it's still a lot faster than any Atom netbook.
I found another alternative, but it's only £345 for the next 2 days:
Although the specs are similar from kurosagi01's suggestion, this laptop has a new, Phenom II based cpu which should be a bit faster and provide more battery life. It also has a slightly faster and newer IGP from AMD, the HD4200.