A cynic might suggest that this is a sneaky but deliberate attempt by the government/MoD to discredit the 'Iran 15' - and in some eyes they are already regarded with some disdain - and by extension discredit the whole UK armed forces. It will make the next defence cuts so much easier to push through. Lack of equipment? Who cares. Shoddy medical care? Well they can afford to go private now surely.
Whatever the reasons for this decision - rest assured it has nothing whatsoever to do with the welfare or wishes of the 15.