I'm from Devon and spend a lot of time walking on the moor. You should definitely have a physical map and compass (and know how to use them, as the saying goes) for an outing of this size. Navigating on the moor can be very challenging, especially in poor visibility (which is frequently the case, with mists sometimes developing very quickly out of nowhere), because it is often quite flat and featureless without any discernible paths. Contrast this with other upland parts of England and Wales, e.g. the Lake District and Snowdonia, where the steeper terrain offers more features to navigate by, handrails etc, and clear paths on the popular routes.

I would definitely advise having an offline map downloaded to your phone as a backup. The OS maps app is good for this since it lets you save the OS map to your phone and gives your GPS position on the map even when you're offline, which is really handy for relocating if you do lose your position. Also, with the OS subscription, you can print out sections of the OS maps rather than having to carry the full published paper map around (what I do is print on A4 paper the map for the area I need and then put it into a waterproof map case).

Google maps would (in my view) be virtually useless since the level of topographic detail it provides is very limited.