WEBF,
If you asked the question at a Gymkhana, I daresay the big defence 'must have' would be the cavalry. In Devon, it's clear that the Royal Navy (and carriers) seem more valid, viable and relevant than they do to others, in less inbred areas of the country.
(Just so you know I'm half-joking).
With regard to PrOOne's assessment of the difference between Tory talk and Tory action on defence spending, it's a matter of historical fact that the Conservative Govts since the war have been responsible for some of the biggest and most damaging defence cuts since 1945. To admit that in no way makes one a New Labour stooge.
With regard to the supposedly catastrophic gap in our defences which will be left by the early retirement of the SHar, you should stop beating your chest for a moment and consider the following.
1) Britain cannot afford a full 'Golf Bag' and must concentrate its resources on those capabilities which are most relevant to today's situation. I'd suggest that that infers light, flexible, mobile forces which deliver capability affordably. I'd suggest to you that the relative lack of use of the RN's Sea Harriers in recent conflicts and campaigns indicates that they are perhaps a little bit more specialised and 'niche' than other air power assets.
2) Whether you or I like it or not (I know that you don't, and I don't much either) today's armed forces are tailored according to the defence assumptions drawn up by the Government and the Chiefs of Staff. It is no longer an assumption that we will conduct major operations autonomously. Since our allies (the US, France, Spain and Italy) can all offer carrier-based air defence (in those rare occasions that land based air defence cover is not available) the decision has been taken that a temporary loss of capability in this area is an acceptable risk. In my view, it's not radical enough. With ever-shrinking budgets I'd have got out of the carrier game altogether, and spent the money on cheaper, more useful land based assets. But they haven't done that. They've decided to retire an ageing, increasingly expensive, increasingly difficult to support asset about six or seven years before it can be replaced. The RN will remain in the fast jet carrier business through the Harrier GR7/9. Retiring the Shar will provide huge savings in logistics and support costs, at the expense of temporarily relinquishing a capability which hasn't been uniquely needed since 1982 (and then only in extraordinary circumstances which would not be repeated). They might even screw some money out of the Indians, Thais or Japs for these ancient airframes.
I'm not saying that the SHar hasn't proved itself capable. In the Adriatic its radar and AMRAAM capabilities drew praise and admiration. But only in the Falklands did the RN need the SHar because no-one else was there to provide an alternative source of fleet air defence.