Re: Cost is the smaller concern
"it will become difficult to hire a different company to maintain it."
Why on earth should that happen? The source is available. The upstream, Debian>Ubuntu is already maintained. If NHSbuntu makes any more widely applicable code contributions - such as smartcard ID - they can also be passed back upstream and integrated with it. So what you'd be looking at would be a collection of userland add-ons which could be maintained separately. You (a) wouldn't need a single company to do that and (b) iit could be competitively tendered. This is a different world to what you're used to in Microsoft.