AA - On a purely financial basis you are wrong. Yes the state does subsidise the private education sector slightly in comaparison to some other private industries; but they still make a net benefit to the economy before you factor in not having to edcuate the 500,000 extra children.
Teacher training? - How is this different from accountancy training for people to work in the city or engineering degrees to enable manufacturing jobs. The state chooses to create a capable work force to the benefit of all. It matters not where these people end up working. Also it's a tiny number. (Circa £750 - 500 million lifetime cost of funding all the teacher training for teachers currently teaching in private schools, thats £10-£20 million per year to fund new teachers entering private schools, assuming that all the teachers have undertaken a state funded PGCE or similar, a lot of them won't have)
Cost to schools of experienced staff leaving? - Again this is negligible since pay isn't all that different between sectors and the state has the benefit of good pensions. You could argue that the private sector causes pay inflation in the state sector but then you would be arguing for a reduction in teacher pay through abolishment of the private sector. Would that be a good thing? That would just lead to a lower quailty of teachers throughout.
Tax on donations? - Very spurious point. If some rich person has money to donate, maybe for tax efficency reasons, maybe just for alturistic reasons they will donate it to somebody. Mostly this will be a charity the performs a function that in a more left wing society would be funded by the state. (look up the french atitude to charity, very interesting! We are one of the most charitable nations on earth)
There really is no doubt that getting rid of private education would be very, very costly. There is no way this cost could be recouped from taxation without affecting everybody, tax may be progressive but it's not that progressive. Just look at the controvesy surround teh 50p tax rate and that didn't even generate enough cash to pay for 1/5th of this extra education.
You could argue on idealistic grounds or even on some majestic macro econmic ideal where the country would benefit for a more equal society leading to greater prosperity etc. But on a simple cost comparison you are well wide of the mark.