The fact that in some other European countries they have a different attitude to nudity doesn't mean it was ok to make (as in enforce) girls shower naked together. That's precisely why it was so humiliating, because adults in the UK would never ever have done this, people covered up their bodies except for teenagers, which was even worse as we had all gone through puberty/comparisons were rife.
In our case, we didn't all go in together naked, we had to lift up the long shower curtain which then exposed all the other girls, then throw our towel over the top quickly to show we were showering and indicate we were using that cubicle- this then led to girls who weren't quick enough getting exposed, and everyone shrieking when the shower curtain was raised.
Ugh, that and the knicker shorts which were knickers. Really humiliating at a time when you need respect and kindness towards growing (or grown) bodies. No period diary thank god, we were told periods were no excuse for missing PE though, which in my case wasn't true as I quite often vomited.
I'm sure our obviously lesbian PE teacher was uninterested in us as were the other teachers I never got any untoward vibes. However, people who did have an interest in under-age girls and boys would have had a field day. By making it fine for teachers to break social norms about nudity, and to wield power over teenagers in a really direct way, this then created conditions in which on the rare occasions people were abused/felt violated/were in ambiguous situations, they couldn't speak out. Some teachers undoubtedly got off on seeing lots of teenagers naked, because we know these people exist. It's a bit ridiculous to claim this is impossible, given the scandals now coming out about coaching sports.