Dear Brian,
Of course the vast majority of Harrier sorties have been flown from conventional bases. A wing of Harriers operating in the Field causes as much damage (and annoyance to civilians) as several squadrons of tanks on manoeuvres. As in the case of tank training, Harrier field training could only be carried out after bribing landowners with large amounts of dosh, and we only did this 3 or 4 times per year on specific exercises. This was how off-base training was conducted in West Germany, for example, during the Cold War. In wartime you can do what you like.
Cold War Harrier Field Deployments in Germany were a sight to see. We used to deploy 36 aircraft on 6 well-camouflaged sites, each pilot flying up to 6 missions per day with cockpit tasking. The Wing would Surge Fly up to 300 sorties per day.