Last year we went to Next for petite jacket and trousers for DD. She was size 6. It was skirts that were more difficult.
In the end we bought a black pencil skirt from M and S in school uniform section. Think it was age 13!
It wasn’t a fun shopping trip simply because the choice was so poor. I think H and M probably had some stuff too, but the styles didn’t suit her.
And yes, in summer the female teachers look like they mostly wear summer dresses and skirts and in winter, skirt/dress/trousers with a cardigan or possibly a contrasting jacket - but not a suit to be seen really.
I think it’s a means if control and also a way for certain schools to suggest they are better than others which allow jeans and casual clothes.
When it’s really restricted, it ends up with girls expressing some kind of rebellion/individuality which teens always need to do, via their skirts being ridiculously short and tight, or their tops being skin-tight. It doesn’t look smart and grown-up at all.