Yes, the matter under discussion got changed from ACARS to GPS, rather confusingly
The two are totally unrelated.
Perhaps someone thought the Airbox product has some kind of online connection. It doesn't. It is just a little box running the CAA VFR chart (as you would get with say Memory Map). But because the aforementioned chart is no more than an image (a picture), there is an additional database layer containing the airspace shapes, and there is software to check for lateral and vertical (using GPS altitude) busts against these.
VOLMET is a good point. It isn't something much known in GA and I have never used it. It probably gives a good general idea of weather, but it probably won't give you localised info like whether a certain (small GA) coastal airfield is getting fogged in.
Englishal - back to the GPS stuff; I often wondered whether the CAS busts data so often quoted by the anti GPS crowd (i.e. that busts are done more or less equally with and without GPS) is actually bogus. It looks like it was all along, because NATS would not be doing this if GPS made little or no difference.