Can I step in?
The difference in fees for many schools will be driven by the "status" of the school, its financial health and the sort of stuff they spend their money on.
So Hornsby House is a charity and so is Finton House. Although they charge market rates they don't look to make a profit. That's not to say they don't make a surplus, but that's not their over-riding objective.
Thomas' is a business owned by a family. They may have education as their "product", and they may do it very well, but at the end of the day money is distributed to shareholders. That may mean that these schools borrow money for capital expenditure, have interest payments to make and generally tend to have to charge more as someone, somewhere, is using it as an income stream.
The profile of the school is also important. I don't know PG very well but I suspect it's not a school that feels it has to have the "best" of everything. I'm not saying that their facilities aren't great or their teaching isn't amazing but there are some local schools (*cough*) who appear to have an attitude that everything they have has to be the very best, from the flowers on the drive to the pointing on the bricks. Personally I think that approach is a little OTT and its expensive.
Hope this helps
