Good points on both sides.
Getting technology into the hands of the clinician and subsequently getting them to use it is hard enough with systems they know; the vast majority know Windows, a minority know Mac, very few know Linux. it's all very well and good saying "use FOSS" (horrible acronym) but the training costs would be phenomenal; IT training departments would have to be expanded/inflated with contractors.
Some systems are so widely used that you can't do it in a phased approach. Using a previous example, how do you integrate your [big corporate brand] PACS (digital xray) system and you new "free" PACS systems with bi-directional messaging so that windows users see what the Linux users see? You simply *cannot* have clinicians looking at the same diagnostic image/information and seeing 2 completely different versions of the information because it syncs over night.
A consultant struggling to use, for the sake of argument, Linux based IT systems would waste more cash in a single day than the cost of a single windows license+CALs. I say waste because they will give up, refuse to use it and potentially cancel their clinic.
The majority of NHS England applications (SPINE) require IE to work. Granted, you could use something like Citrix to deliver an IE environment to the user on a Linux desktop but there again, you need a CAL and a Citrix license per user (recurring cost), potentially platinum level if you want to used advanced features; this is all costly. The same could be said of any application that "requires windows" so you get better ROI but you need separate server pools for your applications which have different requirements.
Fact is, no single organisation can pay a development company to create all the applications it needs to run on Linux (again, for the sake of argument). NHS England/Government could as a whole potentially. Please tell me of an EPR Supplier that is "FOSS" compliant? I know of 1 EPR supplier who is potentially making their code base open source, but it runs on windows with SQL server.
I think at the end of the day, the costs are roughly the same.