Flatus - The Harrier GR.5 was designed for attacking Soviet tanks on the European plains. The gun whilst desirable for that role didn't really stop the game. The current situation in Afghanistan is very similar to events in Vietnam. Effectively lightly armed groups which can harrass our forces quite effectively and tie down large numbers of troops just to defend our positions.
Now by all means you can deploy weapons designed to destroy tanks but when the enemy is a matter of yards away it's not really sensible.
A gunpod could be added to the Harrier but you are still in the position of a pilot sat in a carbon fibre cockpit at low level - hardly desirable!
If we are going to engage in these ventures abroad it's really time we had a long and sensible look at what we operate.