Eco outfitters (eco school uniforms) sorry I can’t get my link to paste. Their sizing is generous so size down if needed, the shirts are a lovely fine cotton.
John Lewis used to do a pure cotton shirt, a thicker cotton than eco outfitters, but cheaper, as did M&S. I need to get new ones soon too.
Out of the ones I have bought the Eco outfitters ones are the best quality but the others are perfectly good too. The John Lewis ones for girls are slightly fitted which is good on my skinny dds.