I've only read half the thread, so apologies if I've missed stuff.
I didn't go to boarding school, but I have five cousins that did. Most of them enjoyed it but one of them has since said she doesn't think it was money well spent. When you look at what they have achieved it is good, and they are all doing well, but no better than those cousins who didn't go to a very expensive school.
Only two out of the five went on to university. All are in jobs that are easily comparable to those cousins who didn't go. Similar amounts of money are being earnt, so it hasn't led automatically to better chances of uni and a higher wage.
One was so desperately miserable for the first two years that he cried down the phone every night. I know, because I was on the other end of the line (I'm much older). He did then go on to enjoy it but I hated being the one to explain why it was important he stay there when he was so unhappy, and when I didn't believe it was important. He did none of the extra activities etc either.
Of the two that went to uni, one flunked the first year. He was so used to having an organised prep time, etc, that far from being independent, he couldn't cope without that level of structure. He pretty much coasted at university, because no-one called a register at lectures, or chased up missed pieces of work, so he felt he didn't have to do them (if it was important, they'd chase him, like school had, right?) and then failed the first year. So when people talk about independence, do they mean not relying on a parent, which isn't the same thing at all as independence if the kid has simply learnt to rely instead on a strict school timetable.
I worked with a girl who also went to boarding school (Dad was in the army). She loved it but had no relationship with her parents because of it, she had to remind herself to call them, because she had grown up without really having that contact, or the shared memories, which make a family.
When I look at all that, the experiences of the people I know who went, and where they have ended up since, I can't see it's worth it unless it is to provide stability (eg, army child whose parents are always moving around). And since I don't work in a field with an old boys network, I don't really see the benefits of the contacts argument either.
So I wouldn't accept that offer. I might, at a push, accept the offer of a private education if, and only if, the local secondary schools were really crap. But not boarding, at all, ever.