We are on quite a tight budget, usually. However, this year, my ILs and my parents and my siblings have all said they are clubbing together and getting her an IPad mini + a £15 iTunes voucher and a case.
We have a budget of £30 (DH and I) to buy stuff for her with. She is ridiculously grateful for anything and everything we give her, unlike the other DC!
She loves techy stuff, but I guess that has been covered my/DH's lovely family. 
I was thinking of buying her a ticket to Fly 360° at the Science Museum (£12).
Then we have £18 left.
We have got £4 worth of small gifts hung on the tree (chocolate, candy cane, some pens etc; and some hair chalks).
But what would a 12yo girl love for £18 (preferably more than two things!)? We are stuck.
She loves music, going out with friends, clothes, food, relaxing, boys, animals and history. 