Bumbalo yes house prices are generally higher in catchment of good schools, but the streets arounds Tetherdown are mainly Edwardian, large and very expensive, (£1.2m + on average) of course there are flats and smaller houses on roads that are a bit further away, but I don't know what the catchment for Tetherdown was this year, it did go up when they first went to 2 form entry but I imagine it's still rather small and it is located in one of the most expensive bits of MH.
There is certainly plenty of families with spare cash for extras like holidays, tuition, parties etc. I don't think it's that competitive as the school seems to have a close community (outsiders view mind went to another MH primary), but it is probably considered the poshest MH primary.
I don't think the non-uniform thing is a problem although it probably has a bit more of an effect on the girls, but if you then send them to Fortismere which many ( but not all) parents do, they do not have uniform either and that isn't an issue there either (personally I think it's mainly parents who like uniform).
Personally having children in both private and state, I think you need to decide what you are looking for because as I said before you seem to be trying to compare two different things.
I moved to MH for schools before my first DC started reception, knowing it was a great choice for schools both primary and secondary, but it was also a lifestyle choice, as I knew from others that MH has a great sense of community and that was something I wanted to be a part of for me and my children as I was at that point a SAH mum and wanted to have other mums around me and I can say MH definitely delivered on that point.