Yes, I'm all for uniform.
The fact remains that research has shown that a smart uniform can be part of a strict discipline policy and is considered a mainstay in the 'how to turn the failing inner city comp into an achieving Academy' handbook.
Without this descending into a 'Class Issue' the reason DS1's school have an 'informal approach' to uniform- though the rules are strictly applied! - is the school in question couldn't be in a more leafy suburb. I think the point of the strict, smart, sharp Academy (1st tranche) uniform is to instil a feeling of belonging, of being part of an institute in which you can hopefully feel pride. This might form an island of security and calm amidst the sea of chaos that engulfs the rest of their lives. In the instance of DS's state secondary, I reckon the DCs there have strong self belief and a strong sense of belonging, security and pride, being valued in all aspects of their lives. The school just provides another service (their education) which the vast majority of them are well placed to access fully and engage with in a disciplined manner without the need for the visuals of a smart uniform. More's the pity!
As for "A lot of countries that outperform us in academic achievement tables... have no uniform". No they don't necessarily 'outperform' us. It's just we English, in particular, enjoy indulging in a spot of self-flagellation. We are masters at doing ourselves down. The reality is can you be SURE those foreign DCs are being measured against the same criteria as ours? Or have they just achieved the locally agreed standard, however high or low?
You will always hear on those 'Move to Australia' TV shows, that, along with the mention of 'the fab blue skies, the endless sunshine, the great Outdoor Lifestyle' will come mention of 'the great schools', like they are as 'given' as better weather. No they aren't. It's just that Australians have a boundless self-belief untainted by doubt.
The French are suffering a crisis of direction within their education system, recognising its prescriptive and dictatorial style doesn't produce the thinkers of the future.