Another one in agreement. I have a very tall 9 year old who usually needs 12-13 in tops as she has a long torso and 11 in bottoms. From when she was about 6 I've found buying stuff quite a trial because so much of it was inappropriate and/or vampire-esque or plain vile.
Sainsbury's is generally very good for that age group - especially as they quite regularly have 25% off clothes but the range carried varies hugely between stores. Tesco / Asda is hit and miss .... the occasional nice T shirt but lots of nylon, lots of black, lots of very short shorts (why are little girls' shorts so extremely short FFS ?!?) and very short skirts dominating their ranges.
Someone mentioned Debenhams - have found they often have very nice, and appropriate T shirts in a wide range of colours (and not just bloody black or cerise) and designs but are particularly fond of sequins (IMO) which aren't always especially practical. But much better than most of the high street - only complaint is that Debenham's prices edge towards being on the expensive side for me, being that little bit than I'd prefer to pay but they do have decent sales with generous reductions so it's worth looking to stock up for next year ... have had some very nice cotton dresses from the sale.
I really really like John Lewis - as do most mums I know - but again it's on the expensive side really for anything except special occasion outfits for me. Wish I could afford to get most of her stuff there - it's not identi-kit and again, has a good range of colours and different fabrics.
I used to think younger girls' H&M were good - but last time I looked, the older girls' stuff was dark and tarty so have pretty much ruled that out.
Next is hit and miss IMO - have had some lovely things, but you look at others and think what on earth were they thinking. Their nightwear and underwear is usually nice though.
Oh - and the BHS older girls' range is foul and plain weird. I still have a Xmas present gift voucher for DD and haven't yet found anything either of us want to buy. It just seems .... eerrrrr ..... odd, with strange designs and cheap scratchy fabrics.
So ...... it's a lot of hard work to get what we want/need because there is so much "unsuitable" stuff about. I realise it's all subjective and that the stuff I hate must be getting bought by someone but think there's a definite gap in the market for a John Lewis-like range - but cheaper - and more widely available on the high street.