“School trousers are horrible” As opposed to school skirts which are a much-loved item of clothing 
I can’t really see that this is something to get worked up about. As long as they don’t implement it overnight so that parents who have already paid out for one set of uniform don’t suddenly find it unusable and have to pay for another. It’s just a straightforward change in uniform policy, making it (more?) equal for boys and girls. Sounds sensible to me.
As to the give them a choice argument - where does it end? If your child only likes to wear strappy tops should they also have an option for that? What about if your child only likes to wear flip flops? Jeans? Pink clothes? Cycling shorts? Should they all be a choice too?
It’s a uniform, by definition it’s not free choice. Your children can wear whatever they want outside of school. In these schools they’ve made the perfectly reasonable choice to just have trousers.