Navigon, I work in sales and drive all over the UK. It's accurate, although it grossly under estimates your travel time. Generally speaking at a fifteen minutes on to journeys that are over 1 hour in duration, and an extra fifteen for each subsequent hour to be on the safe side.
The speed camera alerts are ok, but be weary of the POI's. As with all Sat nav apps they keep ones that are wildly out of date in the app so they can falsely advertise how many they have in their database.
I use a sat nav daily to find clients, and I'd be totally lost without it.
I should probably add that the GPS on your phone can cut out when you're driving in central London, the buildings block out the signal. The app also has a walking and transit function, although I've not used the latter