It's worth pointing out that the only variance it can make under the existing powers is to _raise_ income taxes - this is why it's never been used. Doing so would be political suicide and see a population drain as those able to would quickly move "next door" to an England with lower taxes.
The Barnett formula determines the size of the block grant, not the spending of it. The Scottish Government spends the grant according to its priorities. At the moment these include: free healthcare (including prescriptions), free higher education, and free care for the elderly.
For people not in the UK: people in England pay for each item on a prescription. The charge is currently £8.05 per item. You can get discounts if you need multiple items. There are a bunch of exemptions - people with a thyroid problem for example - which mean that about 90% of items are free.
I would strongly disagree with the CDF being "excellent" – it's basically money cut from other NHS budgets and repurposed for expensive individual cancer therapies.
At the very least, an expansion and improvement of the existing equivalent system in Scotland (individual treatment request) would offer a better solution for the need to fund individual specific treatments that haven't been widely approved.
The Barnett formula determines the size of the block grant, not the spending of it. The Scottish Government spends the grant according to its priorities. At the moment these include: free healthcare (including prescriptions), free higher education, and free care for the elderly.