"At the point you would be paying the fees, all of those are imponderables. "
Eh? If you are studying to become a primate scientist in the Congo, or whatever, then you are unlikely to pay back your loan - it's far from imponderable.
Some people do plan to become SAHMs, not everyone to be sure, but it's a real thing.
To be quite clear, people are being quite dramatic about the loan, but let's say you borrow £18k a year, then over a 3 year course the total real cost is ~£3,200 (ignore the 6.3%, it's 3% + RPI, so in real terms it's 3%) but only if you are going to pay it back.
If you are not going to pay back the loan, then the £3,200 is just a meaningless number.
So yes, it's broadly speaking very stupid to pay the fees up front given that if you have £60k sitting around then it's likely earning 4% or so in some kind of investment, so the real cost could be less than £3k. And then if after graduation your child decides to do something low-earning, then why would you pay it back? It's money down the toilet. Better buy them a house or something.
Basically the cost of borrowing is ~5% of what you borrowed in total, over a 3 years course BUT once you graduate it doesn't grow at all unless you are over £25k. So why not, as a sponsoring parent, defer payment until that point?
I think it is a good way for affluent grandparents to spread their money and reduce the risk of the family losing 40% in inheritance tax
It depends - the advantage is that it is upfront, BUT as a grandparent a £9k student fee payment would attract IHT if they died within 7 years. It doesn't change the tax position when compared with other forms of financial help to a child. The issue, again, is that whereas IHT may be seen as a gift to the government, so too is repaying a loan that doesn't need to be repaid.
Absent the somewhat imminent mortality of grandparents, the best time to repay a student loan out of savings would be after graduation when the graduate was already earning £45k+/year, because that would be the point at which the 6%+ interest actually starts to bite, and it was clear that the loan was likely to be repayable.
(As above, with a £60k debt & inflation of £2k/year, the 9% repayment rate means you are only even paying the inflation when you earn above £47,222, so still not actually repaying the loan, but at the point that you had such earnings you could at least have a much better-informed perspective on whether you were likely to be on £60k in 2 years or whatever, so better to pay off the debt now than allow it to grow in real terms)