I've been thinking this for a while, and have been inspired to post by a current uniform related thread, so it's a TIBAT (Thread Inspired by another Thread).
There is one linked pair of schools in our town which do this (the badge says XXX Infant and Junior Schools) and as it's the only infant / junior set up in our part of town, as all the others around here are primaries, I just took it for granted that all linked schools did this (I'm too old for primary uniform to have been a thing when I was at school myself) . I was actually quite shocked when I realised that not only do none of the other linked schools in our town do this, but that some even have completely different colours from each other.
As primary uniform is supposed to be as low cost for parents as possible, AIBU that it should be part of government policy on primary uniforms that linked infant & junior schools should have to share a uniform?
FWIW, I understand that this can't generally be applied to first and middle schools in areas with a three tier system, as there can be at least two first schools feeding into a middle school.