There is a local school to us ~ also Catholic/Private/Grant Aided and all the other stuff, that has a similar uniform. It has something to do with TRADITION ~ in other words it hasn't actually seen any need to change for the past fifty years. In the 1950s when I was at school all girls looked like that and all boys wore grey flannel shorts until they were 14.
Yes some parents make the choice on religious grounds, some for the "Values" that the "Tradition" indicates to them and some just to be different but the kids seem to survive despite all.
Oh and School uniforms serve two purposes. They create a "corporate identity" with which the children can and do bond and they prevent any "My clothes are more fashionable than yours" bullying.