I haven't watched all of this yet - Channel4OD crashed part way through - but I can see the different sides in this debate.
When dh was thinking about applying for a new job a couple of years ago, the single biggest factor in the decision was the fact that, if he got it, the children would have to move to new schools and leave all their friends behind. This was a major decision for us, and a big upheaval for them, and we were lucky that we were able to do it with the least possible disruption to their educations - although ds1 did go back a year (we moved to Scotland, and he would have had to catch up two full terms of course work on a very different curriculum). As a child, I hated having to go to new schools where I knew nobody, and I can understand the rationale that boarding school gives the children a stable education.
But I also agree that 8 years old is far too young for this, and I suspect that moving schools is less disruptive educationally (though not socially) before senior school than afterwards, so if I were in that situation, I would consider boarding school at senior school age, but not before.
But mum72's post is also very compelling and makes some good points. When the Glasgow job came up, dh did consider getting a flat in glasgow, and commuting weekly, so that the dses and I could stay put, but we decided that it would put too much of a strain on the family as a whole - and clearly mum72 found that this was the case when they tried unaccompanied postings. Clearly boarding school was the right choice for her children - and I do believe that she isn't deluding either herself or us when she says that they are happy there.
Bottom line, I guess, every family is different, and what suits one family well would be hell for another, and all that any of us, as parents, can do is our best.