What the 'rank and file' get is completely irrelevant to the discussion. GMB and Unite don't represent them and the T&Cs in relation to persons employed by a private company and those of service personnel are not very comparable.
Yes. And no.
Yes, ISS are a totally different organisation to MOD, with their own Ts&Cs etc etc and therefore it is largely irrelevant what both bodies decide to pay their own people from the other organisation's perspective.
But also no when we are constantly being reminded of the role of contractors in the whole force concept. And when one element of that whole force has repeatedly gone above and beyond and has been on a pay freeze (cut) for several consecutive years whilst another element is widely and regularly noted for the lack of quality and poor value for money and yet still demands pay rises, there is a higher level presentational issue here. And in that light, ISS won't do themeselves any favours in the eyes of a large proportion of the serving military who have to live on base and put up with what restricted offerings are available.