FA18F is probably more capable than GR4, but that is irrelevant in terms of acquiring the aircraft, as the requirement being procured against is JCA, which requires significantly more capability in certain areas. No-one, I say again, no-one, is going to get a new-buy aircraft past the EAC and the Treasury unless it meets the future requirement.
For all Boeing's current strapline ("Affordable. Capable. Available Now" - wonder what they're thinking??!!) you are talking about an aircraft for which the first production model was delivered 14 years ago (yes, really) and was developed from an aircraft first delivered in the early 80s.
So - new buy of SuperBug unlikely to get past MoD hurdles. In any case, the line is now ramping down with only 67 E/F and some Growlers on the plot between now and FY2014 when production ends. This also makes the idea of "loaning" a squadron or two of 18E/F and deferring F35C much harder, particularly if F35 is delayed.
The only loan option that might be feasible now would be to consolidate all those on "exchange" into a single squadron (would have to build up with a high US content to start with) and operating FA18C if we're lucky, although there are only a couple of dozen C models at AMARC atm. Kind of a reverse of the Eagle squadrons. Depending on the conditions of the exchange, the UK gets a coherent CV-capable unit in the Orbat maintaining skillsets and the USN (or even the Foul & Most Foreign) get an additional squadron of manpower, although fitting in with the deployemnt and training schedules for a CVW will be very tricky.
UCAVs are nowhere near being ready for operational deployments as part of a CVW yet. There's a reason it's called the X47....