"uk schools are very good at differentiating and supporting children who are at different stages"
I'm not sure it's as simple as this. I think schools are good at differentiating work to suit the level of the child. I think they support children 'where they are'. I think they try to move them on.
I think the really big issue, is that they don't (all) have the time/expertise to work out why the child is where they are, and whether they would be where they are if they had a different method of teaching.
Using the example of my DD2:
She's year 2, August born. Several staff at her school have told me they think she's 'really quite bright'. Yet, she is in the lowest group for maths. She can't count money (despite teaching at home and school). She isn't secure in her number bonds to ten (she knows them forwards, backwards, inside out, but doesn't realise that 'the number bond of 4 is 6' means that '4+6=10'). Can't use number bonds to inform her other maths work. Can't count in 10's to 100 (can reel off '10-20-30-40-50....' but when asked to break it down says '10 add 10 is 20; 20 add 20 is 30; 30 add 30 is 40...')
She's in a middling group for literacy, but if working independently: Doesn't use capital letters at the beginning of a sentence. Uses capital letters part way through sentences (in non-capitalised words). Doesn't consistently use full stops. Doesn't spell well (at all). Doesn't stay on a line. Doesn't use a consistent size of writing. Doesn't write in full sentences.
She has taken from September til now to settle in her Y2 class. She'd become so disconnected that she couldn't write a title from the white board.
However, she is 'bright'. Truly she is. Questions she asks can be quite profound ('why is there no gravity in space if there's gravity on Earth?' 'can you show me a picture of what's inside a frog, because the skin is in the way and I can't see?').
Now I think she has SN, have told the school, they agree (off the record!) but when we talk about her learning in maths (I think she needs a much more explicit teaching method in general, which is why she isn't learning. I also think she needs a much more visual method, eg. numicon, which the school has but DD has never used) they say:
'Oh well there will be other children of her ability.
They are choosing to see her as low ability rather than low attaining.
This happens a lot, from what I can see. If a teacher teaches and the child doesn't learn, the teacher forms the view that the child is of low ability.
I believe that sometimes a child doesn't need to be taught easier versions in the same way, but needs to be taught differently.