"the alternative is to lose their charitable status and have to pay full tax etc. on their assets, investments and incomes"
The tax 'break' is estimated to be worth £200 per pupil per term.
The real penalty is that there is no way to just remove charitable status (this has already been mentioned unthread, but I'll say it again for the benefit of those new to this issue: there is no way to remove charitable status. A charity that is not fulfilling its charitable aims, under the law as it stands, has to be closed down and all assets sold at full market rate, and all proceeds from those sales to be donated to another charity.
So it is 'be careful what you wish for'. If charitable schools were forced to close, the state sector would have to absorb these pupils somehow. Yes, there would be some overseas ones who simply wouldn't come. But in some parts of the country (London definitely) the chaos in state schools as LEAs struggle to find places will be awful.