You can get Hobnobs at World Market. Not sure I've seen the caramel ones there though.
I'd bring back crisps (chips). There are lots of different flavours that you don't get over here. You'll also find different chocolate bars that you can't get over here.
Here's a good list of shops in London that you can get unusual gifts from:
http://www.timeout.com/london/shopping/the-100-best-shops-in-london-a-homeware-and-gifts-3
For young kids try magazines. I get Peppa Pig, Bob the Builder and Fireman Sam for my sons when I go home. WH Smith usually has a good selection.