souper Why assume they're only socialising with other families doing the same as them? My friend's HE child has as many schooled friends as HE.
I was coming from the perspective of living in an anonymous urban environment in a small flat where it is dangerous for children to "play out" on their own or with others and being home-educated. There's a danger that, without forethought, children are rarely in a situation where they are outside of their parents' direct influence or eyesight.
For most children, this starts when they go to school: they are in an environment where their parents are not present, and this requires them to build new skills. We often see this as "socialisation", but, really, it's learning how to navigate a set of scenarios without a primary care giver within an immediate sphere of reference.
For children from tribal or traditional contexts that do not experience a formal school setting, this starts when they are old enough to go out to play on their own with minimal to zero supervision with siblings or other children from the tribe or village: this also requires them to build new skills in learning how to be outside of immediate parental reference. My grandfather and his siblings had free run of an area that was over 150 acres, for example.
My point is that home-educated children in a small flat in Brighton don;t have these opportunities. Yes, they can meet new children from all sorts of backgrounds, but if those meetings are arranged and most likely attended by their parents within fairly strict parameters, when do these children ever get a substantial amount of time to experience and learn how to navigate scenarios without a parent present or very nearby?
Compare the amount of time a schooled or a traditional/tribal child of ten will have spent in environments outside of direct parental control with a home-schooled child of ten that lives in Brighton with no capacity to roam free outside on their own or with other children, and maybe you can see what I mean.
Combined with a parental commitment to western-style extended attachment parenting, this is what I mean when I say there's a danger that the world of both parent and child can become very insular.