Ah, PE kit, how I do not miss you.
I think my junior school (age 6-11) had a school kit in the school colour (maroon! blech) but I have a feeling most people just wore white t-shirts or polos and whatever coloured shorts. I seem to remember they weren't allowed to have a football team logo on them though.
The first secondary school I attended was pretty simple - black bottoms (shorts or trackies) and then a white polo for indoor like badminton and gymnastics, and a black and white striped rugby top for outdoor things like hockey and football.
But then I moved. Oh, the horribleness of the new PE kit! It was all pretty standard stuff really. White polo shirt. Sweater and trackies with the school logo. Netball skirt. The only problem with it was the colour - it was a really, really horribly unflattering shade of sickly turquoise. Honestly, it could have been much worse - after reading this thread I'm very grateful I didn't have to put up with a gym onesie - but having gone from a fairly relaxed school to one where the PE kit was enforced very strictly, it was a bit of a culture shock. If you 'forgot' your kit, they had a bin bag full of spares for you! In all sizes! (It was at least clean...) One of the few times in my life having disproportionately small feet was actually useful - none of the spare trainers fitted me. Shame.
Ooooh, plus, at my second school they had their own pool. There was no 'school' swimming costume as such, but it was specified it had to be a one piece, plain, in dark blue. Which would have been fine - except finding a plain, navy, one piece swimming costume to fit a thirteen year old seemed to be like trying to find hen's teeth. Fortunately they didn't have spare costumes for people who 'forgot' theirs.