Also consider... both the USN & the USMC currently operate the EA-6B Prowler, but only the USN is replacing them with the EA-18G Growler.
That is because the USN will operate their F-35Cs beside their F/A-18E/Fs (Super Hornets), and they need a dedicated ECM/DECM* "radar jammer" EW aircraft to accompany strike packages to protect the SHs.
The USMC is not buying any SHs... they are intending to replace all their F/A-18C/Ds, and all their AV-8B Harrier IIs with F-35Bs... meaning all their fast-jet combat aircraft will be LO, reducing the need for a dedicated EW aircraft.
Additionally, the F-35 has a radar system that has been described as having "significant EW capabilities" above and beyond any specialized self-protection ECM/DECM units in the aircraft.
The ability to use the main radar in this fashion has led the USMC to conclude that an all F-35strike package simply won't need to be escorted by a dedicated EW aircraft!
Thus, as long as only F-35s are operated from the RN's carriers, there is no need for a specific EW variant, as one or two F-35 in the strike package can be tasked as "EW escort", with their targeting info being networked from the other aircraft in the strike package.
The only need for a dedicated "EW escort" will be to protect RAF Typhoons... and since they only operate from land bases, a 2-seat F-35D without the lift fan won't have any problems... nor would the cheaper option, buying a few EA-18Gs for the RAF!
*ECM = electronic counter-measures (jamming);
DECM = deceptive electronic counter-measures (spoofing).
Combined they are called EW (electronic warfare)