I would imagine that Platinum insurance is full coverage, therefore you would be covered even if an accident was deemed to be your fault. However, I don't know what the limits would be in terms of what the insurance pays out for medical bills and so on, since those limits vary from state to state and, obviously, from one insurance package to another. You should call Virgin to double check with them.
Regarding sat nav, what kind of car (in terms of size etc) do you have booked? It's possible some of the more expensive options come with sat nav (e.g. Cadillac Escalade, possibly) or you could call Virgin and tell them you want sat nav as an optional extra. However, it is likely they will charge you for this and, hey, you can always just buy a map.