Joelsnan,
NI is not a dedicated health provision tax. It's just thrown in with all the other tax revenue. What I think people would accept is a tax rise (and. Not just a soak the rich tax, but, of course, the better off would pay more) where then extra money is to be used solely for healthcare.
As to the elderly, the cost of treating them has increased hugely. It isn't just a case of the same cost always having been there. This is a point made not just by the govt. but by all the experts asking for more funding.
My example of the private hospital scanner and the cost of an NHS replacement was not getting at the cost of the hardware itself (though that let are not cheap) but the cost of providing and staffing a full time unit to run it - absolutely huge. If, for example (I'm making up some figures but you will get the idea) an NHS hospital has scanning facilities which cater for only 90% of its needs, it might be much more efficient to take up some available slack in a local private hospital than buy another scanner, build a new unit to house it, and pay full time staff to operate it. I'm not saying private provision of services is always right- but there are instances where it will be.