Our school doesn't allow cycle shorts for indoor PE, no idea why and half the girls wear them regardless. DD has boy's cotton PE shorts from Asda.
Also are you sure they have to be uniform? We have to buy outdoor PE kit and indoor PE kit. Indoor has to be red t-shirt and black shorts. Outdoor can be anything, I bought DD school colours not knowing she didn't have to have them, only to have her in tears because all the other girls were wearing hello Kitty tracksuits etc .....sigh.
Thank god they don't go for the cycle shorts - phewy... better ask around some of the mums with kids already there then - as DD would NOT be impressed if everyone else had hello kitty.. .she loves the crap!!
My DC wear football type shorts in the appropriate colour (boy and girl). I find the cotton ones too stiff and sticky outy. Having said that the infants were allowed cycle type shorts for the girls. The junior school however does not.