from FodPlod's link:
Captain Jerry Kyd, commander of HMS Queen Elizabeth, commented on the initial deployment and the gradual increase in air wing numbers:
“We’re constrained by the F-35 buy-rate even though that was accelerated in SDSR in 2015, so initial operating capability numbers in 2020 are going to be very modest indeed. We will flesh it out with helicopters, and a lot depends on how many USMC F-35s come on our first deployment in 2021. But by 2023, we are committed to 24 UK jets onboard, and after that it’s too far away to say.”
Interesting indeed that the schedule was slower pre-SDSR15. I recall that in public the people responsible for the carriers and UK F-35 were reticent about both the UK's timetable and the USMC participation; I don't think the two stories would have been received well at the time. I believe the current plan syncs with the rest of the "block buy".