It seems that costs of gluten free prescriptions (as per opening post) was incorrect.
Welsh Government denies Tory claim of £20 NHS loaf | Healthcare Network | Guardian Professional
"However, the Welsh Government responded with a statement which says that Millar is making an inaccurate claim because it regarded "each item" as a prescription and not a loaf. It said the actual cost of a single loaf of gluten-free bread is about £2.82."
"The Welsh Government also confirmed that it issues gluten free madeira cake on NHS prescription at a cost of £2.82 per cake."
'Let them eat cake'

- it's the same price!
I do not regard bread and cake as essential for a healthy diet.
So I see no reason for it to be provided on prescription(free).
If you really want it, you can make your own.
Doves Farm Gluten Free White Bread Flour (1Kg) in Tesco | mySupermarket