It could be argued that having an expensive uniform creates a barrier to entry whereby families have to, literally, buy into the school's ethos.
It dissuades those families not prepared to conform to the school’s uniform requirements from applying for places and gives a measurable indicator of behaviour and attitude, which influences Ofsted gradings. Parental attitudes heavily influence pupil behaviour, so in one swoop the school potentially removes a percentage of potentially less-compliant pupils by discouraging their parents from applying in the first place.
In the same stroke, uniform cost also discourages lower-income families from applying for places. Overall, children from disadvantaged families do less well academically so the school attracts fewer children who are less likely to reach the academic targets the school is measured by, and it also needs to spend less money in subsidising pupils from families who are struggling financially.
[These are obviously generalisations and there will always be high-achieving pupils from disadvantaged backgrounds and well-behaved pupils with unsupportive parents, but not, and certainly not perceived to be, the majority.]
Higher Ofsted gradings, better exam results and good local opinion are key factors that parents use when choosing schools, so it becomes a self-fulfilling pattern at next-to no cost to the school itself.
It's not right, but that does not mean it doesn’t happen.