I mean, you can't speak for the entirety of the UK. I was born in the 80's and at my secondary girls absolutely could wear trousers and boys absolutely weren't allowed long hair, and neither gender was allowed to have non conforming hair styles / unnaturally dyed colours.

Guys also weren't allowed earrings, girls weren't allowed piercings other than in their ears. Both genders got away with it if they covered them with flesh colored plasters. Neither gender was allowed makeup, but girls generally got away with it if it wasnt too obvious.

The biggest arguments were around girls skirts having to be down to their knees, and both genders ties having to be 5 stripes long.