Staines, another poster - 5GoldenFimbos - told me that this age group is really into Superdry. If your nieces would be size 6-8, say, then you can get them one of their t-shirts in a size xs either from the Superdry site or from ASOS. Both offer free delivery atm.
ebay has a superdry discount store so worth checking out, its official superdry and run by them so not fakes. i got ds1 a superdry jacket for £29.99 reduced from over £100 and hes very pleased, also a hoody for £14.99 because it had a hole on the inside of the pocket!
other than that, john lewis, fat face, gap. Even m&s has some nicer older girls stuff. Other than that my 13 yr old cousin likes new look and topshop, new look have childrens range as well as adult range and Alice wears a size 6-8 in topshop. You can find measurments on the site of somethign that you know fits them and then compare with the measurements on the site that you have not used before. e.g my ds1 wears an age 12 in boden, so i brought him an xs superdry hoody as they were both a 36" chest measurement.