Engines,
"The F-35C is not being bought to do the RAF's GR4 role."
Yes it is. There have been numerous statements over the past few years that the future RAF fast jet fleet will be a combination of Typhoon and F-35. In the days of the JCA being the F-35B there was serious talk at ministerial level of a wing of 2 squadrons of F-35 type aircraft IN ADDITION TO THE JOINT FORCE LIGHTNING with a unit establishment of 9 ac each and a dedicated OCU to serve as the manned fast jet element of the GR4 replacement.
There is a widely held assumption that the UK buy of F-35 will only be around 30 to 40. This is not backed up by any statements of force levels post 2020 for the Joint Force Lightning OR the GR4 replacement.
There is little likelyhood of the F-35 being ordered in quantity in the next few years, but one of the many advantages of the F-35 is it's anticipated production run with the USAF committing to a very large quantity in the most recent DOD review, and consequently there is no NEED to buy the thing now, we can order it when the defence budget is less squeezed and there is a need for the Carrier based AND GR4 replacement fleet.
While there is absolutely no possibility of a one for one replacement of the GR4 fleet, I think that there is a very strong likelyhood of at least an additional two squadrons in addition to the Joint Force Lightning force flown by the FAA and RAF.