"There's currently a thread in primary where the OP asks whether some subject specialists introduced at Primary level would improve standards...
Resounding rejection. What a surprise! "
I have just read that thread, and the response is actually more subtle than that.
Basically the consensus seems to be that all see advantages of MFL, music and PE.
In KS1 (infants), the consensus seems to be for having a single 'class' teacher for these youngest children.
In KS2 (junior), the thread was much more in favour of specialist teachers, though there was a worry about 'compartmentalising', when so many schools do such excellent cross-curricular work e.g. teaching humanities through English and vice versa, or ICT in Science.
It's interesting, because certainly until recently, I taught my specialist subject (I have an Oxbridge science PhD) less well than I taught most other subjects. Teaching in primary is much less about detailed 'subject knowledge', and much more about pedagogy and about the teaching of skills. The explicit teaching of skills in Art, say, where I have to think about them consciously, came much more easily to me than teaching the skills of being a scientist, which are unconscious / implicit in me.
Even in Maths, the subject knowledge to teach an able 10 year old is well-contained in a Grade C O-level. A teacher with excellent teaching skills, with that Maths background, could teach maths to the able very well. Equally, the children who often need the most skilled Maths teaching are those who struggle. I have Maths up to first year degree level, and teach bottom set. My colleague who teaches the top set, has maths to GCSE only, but her teaching skills when teaching the very able are excellent.
It really isn't the case that 'high qualification in a subject' = 'able to teach it well to younger children'.