Here's a thought (and a conspiracy theory so diverse that it could have originated in Harrods)
UK buy combined with USMC STOVL requirement means that STOVL version is viable.
No UK buy = no STOVL viability = USMC have to have CV version = USMC have to fly of USN Carriers = no reason for USMC to have organic fast air as USN can do the job with their own CV versions
OK I am bored...