Re: Maybe they should consider going over to Linux?
You say:
"if an institution as large as the NHS, with a commensurate budget, chose to use OS 'Z', there would be no shortage of vendors only too willing to write drivers, etc. for their equipment to run on OS 'Z'."
but actually the NHS is not an institution, it is an affiliated group of Trusts, GPs practices, Commissioning Units etc etc, all with their own, often quite paltry, budgets and income streams, often running on their own WANS and running their own organisation specific applications. It's just not as simple as it looks from the outside.
As other commenters have implied, most NHS organisations that I know of have in fact upgraded most of their desktop estate to Win7 now. The problem is generally with "Analyzers", pieces of healthcare equipment; blood tracking devices, pharmacology equipment, CRT scanners for example, running ancient applications that would nevertheless be breathtakingly expensive to replace with something a bit more up to date.
This story is, to some extent a case of mountains and molehills