If I subscribe to the OS Maps service, do I get it across all platform ie. VR too, or will I just get it in the OS Map app?
"No" and "Yes", respectively, as far as phone apps are concerned.
You also get to use it, though, to great advantage on the OS maps website. Strangely, the website does not allow you to view your bought paper maps online, so you really have to have a subscription to see the equivalent mapping there. I don't currently have a subscription but I still find the digital base map useful for its detail (you can turn on grid references and get them by positioning objects under the central crosshairs). A 2D aerial view is also available without a subscription, as are Green Space and Cycling Network overlays on the base map.
If I do take out another subscription it will probably be with Viewranger. I like the app and the website functionality is great, too. Unlike OS Maps, VR seems to have no official way of printing maps from its website. You can always use the Print Screen facility of the PC, of course, but printing to the proper map scale with this method might be tricky!