My friend is dead broke and blagged 2 of her boys into private school with massive bursaries and scholarships.
They are secondary level though and can demonstrate particular skills (sports).
I would steer away from borrowing against future earnings, this could backfire and you may end up having to pull a child out of private back into state, this is way worse than going the other way.
Where I live the infants and junior schools are excellent and I looked at both state and private and decided state was best for my DD. I will review my choice with each subsequent school move in case this changes.
I have a private school down as my second choice as it genuinely was not £10k a year better than the state one.
It is not unusual for a child to go to a state school and then into private in my area.
However, medium ability children who go to prep schools are more prepared for entrance exams than state children and then you could try tutoring. This is cheaper than full time private.
If you are a barrister in training then you are likely very clever and could even do the tutoring yourself. Once you have established yours child's abilities (at age 7 or even 11) you could then look to the private sector and, if your financial situation is not great, ask for bursaries.
Please note that treating the state sector as "terrible" and the private sector as the only option does reinforce the view that they are simply "social clubs" for the children of the rich and provide no other benfit to the wider community. This view, if continued will result in the removal of private school's charity status and many would close.
This is very hard on the private schools who genuinely make an effort to be community focussed.
I spoke at length to the head of the prep school about the children sitting entrance tests at 7 for the local private schools and she told me that the school had a good sucess rate but there had been children "returned to the state sector" which is code for the fact EITHER they were dim and not able to get into the highly selective private schools round our way OR their parents failed to budget for fees ongoing.
Please note school fees are more annually than a mortgage and you have 14 years of them, is the private sector really worth £10k a year more than the local state schools?
If you do decide to borrow there are firms who specialise in school funding although a mortgage extension may be better value, schools give you a discount if you hand over £20,000 as a lump sum!!!!!!!