I wouldn't have thought that ops over Libya require rapid response in the same way that ops over Afghanistan do, due to the lack of "friendly" forces on the ground. In any case, if there was a requirement to "scramble" an asset somewhere due to an illegal CP popping up or something like that, I'm sure that the ATO means there's enough aircraft on planned missions that could be redirected.
It's not the type of conflict that requires a GCAS or XCAS asset on permanent standby, is it? If/when we end up with 10 000 troops in Libya who could fall into a contact with Gadaffi's men at any moment, then yes, a closer base/carrier would be handy. But my impression of ELLAMY is that it's pretty task/target-driven, and there's much more planned targeting than CAS going on.
The problem with this whole discussion is that the 2 ops we're currently involved in are very different beasts, and you can't read across lessons from one to the other. Speed of response is very important in Helmand, which is why Apaches indigenous to TFH and GR4s on GCAS are required, but there's nothing there that the Harrier could do better (unless you want to get into the minutiae of exact scramble times and exact serviceability rates, but I doubt you'd have a huge difference even then.)
Claiming that a Harrier on CVS near Libya would be good because of improved response times is meaningless when it's the Afghan conflict that benefits from that. I think in this case I'd prefer the GR4's weapons fit when it comes to the sort of things we're doing over Libya, even if we did have a direct choice between the 2.