My personal opinion is that we trudge from year to year knowing that our kit is deficient but not actually doing anything meaningful about it. Take the Harrier as an example, great cockpit, some great weapons. But we never bought it a decent pod, and gave up on the cannon.
We have this mentality that some pieces of kit are icing on the cake - when actually they're the difference between the whole thing being great, or not worth bothering with. We also seem to take a completely misplaced pride when we get a bit of kit 'fitted for but not with'. We all know that procurement takes an age, so having the capability on ice, to be ordered at the rush is no comfort at all.
What we really need is the ability to say "No" to our lords and masters when they ask us to go to locations for which we don't have the correct equipment. We shouldn't have to rely on Urgent Requirements to get kit we should have anyway.
In answer to the original question, i personally think that we have a very good CAS asset in the Harrier, but it needs a decent pod, and (arguably) a cannon. I think that these investments would make more sense than using the F3 in the role. I think that the F3 would actually be ideally suited to a SCAR type role, toting Alarm, ASRAAM and AMRAAM, plus a cannon, it would make a very competent 'bookshelf' for what would appear to be very little outlay.