First of all - consider this:
Over 99% of trans-oceanic trade is carried by sea and the United Kingdom is reliant upon maritime trade for its economic security. Consequently, the UK places great importance on the freedom of the sea; it furnishes a safe working environment for British seafarers, cargoes and merchant vessels, is a contributor to national well-being and, in the wider arena, an important manifestation of international peace and security. The RN has a responsibility for the safety of British merchant shipping in crisis, conflict or war, regardless of whether this shipping is operating for commercial purpose or in support of military operations. In many situations, this interest will also extend to merchant shipping belonging to allies. Maritime threats will, in the future, be broad ranging, advanced and unpredictable. Except in facilities such as ports, harbours and smaller, more fixed focal areas and choke points, these threats will be dynamic. Protection must be enhanced by the combined effect of optimised force distribution and dispersal, signature control, co-operative deception, hardening and collective protection, coupled with the inherent mobility of maritime forces. Further, it will be assisted by the increasing co-operation with the Merchant Navy and the more emphasis on a variety of Maritime Trade Operation procedures. Retaining sufficient force protection capabilities against future threats will remain a core operational requirement. From the Future Navy Operational Concept Paper 2001.
It is possible, is it not, that a scenario could arise where a certain regime (lets refer to them as nation X) decides that it wants to attack UK interests (either to gain "street cred" with its supporters or other regimes, or for other reasons) by attacking or harassing UK merchant shipping. For the moment lets make a number of assumptions.....
1. Nation X is somewhere in Africa, Asia or the Middle East, geography means that they are no established friendly air bases that could be easily used.
2. Nation X has Nations Y and Z as its neighbours, but they do not want to get involved in X's argument with the UK.
3. Nation X does not wish to interfere with US flagged vessels so as not to provoke Washington.
4. Nation X has a small Navy, and a small to medium sized air force with MiGs (various types) and helicopters. Some of these aircraft can fire air launched anti ship missiles (a capability that many nations have with Russia and China exporting missiles).
X harasses UK registered shipping to prevent it going through a certain area, and harasses vessels with missile boats etc. To defend UK vessels, HM Government decides to deploy frigates and destroyers, together with supporting RFAs, to area. X then says British warships will be attacked. An assessment of X's air strength concludes that X has the ability to project air power over the area of interest, and could overwhelm or evade ship based defences. Can we still deploy? Imagine the disaster if things go wrong....
If on the other hand, a CVS can be dispatched along with Sea Harriers, the defence of the UK forces in hugely improved and we can deploy our warships to protect our (civillan) shipping. Better still, the existence of this capability may well act as a deterrent and prevent the scenario from ever occurring. By providing this sort of force protection the Sea Harrier acts as a force multiplier and greatly enhances the capability of our naval forces.
Without the Sea Harrier (or a worthy successor) we cannot undertake this type of deployment by ourselves. Nor do we have a deterrent against the sort of act described above. This is the crux of the matter.....
Prevention is better than cure. Deterrance is better than a situation getting to the stage of combat.
The most important lesson to learn in the Falklands conflict is this: If you hope to deter an aggressor from attack, you must have capable, well equipped forces readily available. But above all, you must demonstrate that you have the political will to use them.
Admiral Of The Fleet Lord Lewin.
You may be interested in this US study of fleet air defence and the lessons of 1982:
USN Study