Actually, it's all about what works, especially in the NHS.
In my direct experience, NHS trusts have been burned by attempted windows tablet computer deployments. They have tried them and they _weren't useful_.
They were completely crippled by the windows UI, and locked-down security using the domain security paradigm was part of the problem. The IT department thought they were neat, but nobody on the ward bothered using them. The bulky, heavy, battery intensive form factor and Atom powered sluggishness didn't help either.
The Metro interface looks like it goes some way to solving the UI issues, so long as the apps follow, which they probably will. But the tablet test platform based on Corei5 is a sad joke, and the duality of the full interface and Metro on the same machine bodes ill for use in a clinical setting.
In stark contrast, a number of Australian hospitals (also in my direct experience) are using iPads, and liking them a great deal.
Yes, if and when real tablets with comparable performance specs to iPads and the enterprise (including hospital enterprise) apps designed for a touch UI to match arrive, there will be pressure from network/domain admins to deploy them instead of iPads. But given the woeful history of windows tablets in places like the NHS, I expect support for this to be lukewarm even within IT, and downright hostile on the floor. "Lets replace something that works, is relatively cheap to buy and maintain, and is accepted by our end users, with something that was expensive to buy and maintain, and rejected by our end users the last two times we tried it" is not a proposal I would be advising our executive to support...