The estate is defined as the property to which the deceased was beneficially entitled at death.
Has she paid away the £75K to the school or merely agreed to do so?
This might also be a timing issue, as to income, and you will doubtless have looked at this, but as she was paying the school fees anyway, I assume, is it not at least in part also arguably “normal expenditure”, albeit in advance and perhaps also out of capital not just future income? She was not undertaking someone else’s liability:
s. 21 (1) A transfer of value is an exempt transfer if, or to the extent that, it is shown—
(a) that it was made as part of the normal expenditure of the transferor, and
(b) that (taking one year with another) it was made out of his income, and
© that, after allowing for all transfers of value forming part of his normal expenditure, the transferor was left with sufficient income to maintain his usual standard of living.
I would agree that there might be a little difficulty in showing that the deceased was able to maintain a given standard of living post decease, even if devout, but there may be a part of the payments which could benefit from the exempted property régime, either for the year of payment, or from year to year, if normally paid out of income.