Vortex,
We could deploy more than 7 cabs to Afghanistan!!!! The issue here is that for a variety of reasons (primarily cost and personnel footprint), PJHQ have capped the deployment at that number.
However, the GR9 fleet is also a small community and the numbers of available aircrew has actually reduced since the RN took over half of it. They are required to remain familiar (if not current) with carrier ops, as well as conducting trg in other activities which may be called for. When 20 Sqn are taken out of the equation (conversion and weapons instructor trg), the normal rotation plot leaves very little flex in the Harrier force.
Why don't we train more aircrew? For the same reason the Army don't have more CSS and CS assets: cost. Plain and simple. However, the trg system is maxed out trying to train the naval pilots required to bring the Naval Strike Wing up to strength.
The deployment of GR4s has been examined but again, the cost of deploying a more labour intensive asset into theatre was deemed prohibitative. Likewise, it would be difficult to fly fast air missions from elsewhere in the Gulf or Stans (or for that matter a carrier) but the RAF tanker fleet is pretty broken.
That's why Typhoon should be deployed next year to enable the GR9s home for some much needed operational rest of airframes.