I'm glad the research looked at comparable weekly shops rather than just comparing the cost of e.g. normal pittas and GF pittas.
The high price of core GF alternative products (bread, cakes, flour, cereal, biscuits/crackers, pasta, oats etc., as well as GF versions of prepared foods like fish fingers or pies) is only part of what makes it more expensive.
(And of course some people are apparently still able to get some of those on prescription — I got blanked every time I asked about it, though, even though a limited amount is apparently still prescribable in my area).
But even leaving those aside, the restricted choice in other products — due to some having gluten-containing ingredients or may-contain warnings — means that budget-conscious coeliacs often don't have the freedom to choose from the full price range of products.
For example, if you're in Sainsbury's and want a jar of mustard, some of them contain wheat flour, and some of them don't — and the own-brand value range one is one of the wheat-containing ones. (Though, oddly, in this case it's the same price as the standard-range own-brand mustard. Anyway.) Or again at Sainsbury's, if you want to buy beef meatballs, the chilled ones use non-gluten-containing filler and are safe, but the slightly cheaper frozen ones (which are also easier to use a few at a time, saving money) use wheat flour.
Or there have been times when all the versions I can find in the supermarket of a particular ingredient I want (like popping corn, dried papaya, or some specialist flours) have had may-contain warnings, so I have to order them online for delivery, which invariably costs more (unless you're REALLY into dried papaya and want 5kg of it). Then sometimes it turns up with a may-contain warning that wasn't mentioned on the online listing anyway 🙄
It's often pence here and pence there, but it adds up.
And all this is mentally wearing — when you add to that all the extra label-reading involved every time you go shopping, and extra mental effort involved in thinking about what meals to make when you have hard restrictions, it doesn't leave you much mental energy left for budgeting.
Plus, there's the psychological impact of picking through all the things you can't have to find your safe groceries. For me, at least, the scrupulousness and self-control and occasional disappointment or frustration that's involved leaves me far more vulnerable to thinking, "Fuck it, I'll treat myself," when I do find something I want and that I can safely eat — even if it's more expensive than I would've previously considered acceptable.
I think campaigning to retain prescription GF foods has been the wrong tack to take, though. Providing GF foods on prescription through pharmacies is expensive, bureaucratic, an extra workload and storage imposition on pharmacists, extra work for doctors approving repeat prescriptions, inconvenient for coeliacs, and an unnecessary duplication of supply chains that supermarkets already have in place. You could also argue that it's othering and pathologising to get your food from a pharmacy, if you're into that kind of analysis.
IMO, where special diets are medically necessary and can be catered for by products stocked in ordinary supermarkets, and are significantly more expensive than a standard diet for whatever reason, patients should be provided with vouchers, or probably just money, with a value based on the cost difference between a typical standard shop and a typical medically-compliant one, using research like this to inform it.
I haven't costed it or anything (and wouldn't know where to start) but I wouldn't be surprised if you could set up a weekly/monthly financial payment to each diagnosed coeliac (and others in a similar position), and, after initial setup costs, end up with a ongoing cost (including admin) that's less than the current total cost of providing a meagre supply of prescription GF products to a small number of coeliacs. And potentially save money for the NHS by reducing the number of coeliacs developing complications through not being able to afford a compliant diet.