Paul,
No one is disputing the capabilities of the RAF in terms of air defence. The RAF are massively capable in this area, being able to deal with most threats out there with the possible exception of the SU-30MK series which scored a hugely impressive kill ratio in this years air combat operations against US F-15 and F-16's.The fact is though that UK military operations now take place in thatres where air superiority is guaranteed and ground attack aircraft are much more useful assets. This may not always be the case and the increasing threat from a pointlessly reassertive Russia is one of the factors to be considered.
An interesting topic this is turning out to be. The Falklands conflict is an extremely interesting one in terms of military strategy.
With regard to the exclusion zone you mention, this was not rigid when it came to ROE. Any target inside the zone was fair game. However, cabinet approval was given for the sinking of the General Belgrano- a turning point in the war, even though it was indeed outside the exclusion zone. This resulted in the Argentine Navy heading back to port and all air operations coming from land bases. This incident is not classified.
The Brtish strategy was to deploy all shiping in task force formation, including Aircraft Carriers. They were mainly deployed to the North and East of the islands, the opposite side to where a returning Skyhawk would be. The Harrier GR.3 was extremely short legged and needed to be within a short hop of the islands to be useful, even for close air support.
An A-4 heading away from the islands would have been travelling West/North West and over water. There would have been no reason for a short ranged GR.3 to be in the area and it would not have had the legs to intercept a similarly paced Skyhawk heading away from it.
In any case, a combat aircraft returning from an operational mission regardless of it's position would have fallen well within the ROE and would therefore not need to be classified, but the fact remains that there were no UK aircraft deployed on the mission with sufficient range to either follow and engage it outside the exclusion zone, or indeed to be on a CAP mission in that area.
I can think of one or two scenarios where this could have been possible, before the official commencement of hostilies, an A-4 on a scouting mission could have been happened upon by a Harrier and shot down as an opportune kill. Remeber the A-4 was the longest legged aircraft in the Argie inventory due to the fact that it could buddy refuel. However, there would be a record of an Argentinian loss at the time to support this. Another factor working against this was that the Argies used Neptunes which could detect the task force at long range, outside missile and at the edge of Harrier range.
I suspect there is more to this than either of us know and it would be worth further investigation. The exclusion zone reasoning by itself just doesnt add up.
Rgds
TM