If she wants "character" (like Barbie, Groovy Chick, etc), like my son did, they are in Argos. But are made of hideous poly-cotton mix which made him sweat so then I went and soend another £40 on pure cotton Designers Guild one from local department store.
Other, more reasonably priced cotton duvet sets can be bought from www.boden.co.uk and www.aspace.co.uk (sorry can't do links), as well as The White Company (although I've had a nightmare time with them not delivering things they;ve charged me for)