Presents under £20 that Santa is bringing my 9 year old DD this year:
Telescope - £10 from The Works
Jacqueline Wilson/Enid Blyton/Dork Diaries/Shakespeare book box sets - various prices from The Works.
Nerf Rebelle bow - I picked DD's up for only a few quid in a sale. You can get them really cheap!
DC Superhero Girls Dolls - they're often falling to around £15 on Amazon.
Mermaid Tail blanket - I ordered DD's months ago from America for around £11. You can pick them up on eBay and Amazon for less than £20 for certain.
Worry Monster - around £10-ish from The Works. There are always discounts flying around so you can pick them up even cheaper. Think DD's worked out at about £7 in the end.
Smiggle stationary - quite expensive for what you get, but their website is constantly offering BOGOF deals.
Lottie dolls - keep an eye out for offers. I got a doll free when I ordered one.