The 2023 date is puzzling, still - but it may have to do with the USN's own schedule for getting operations spooled up. Their IOC is ten carrier-deployable aircraft, which is not a lot for trying to train the RN as well. And IOC is not until after IOT&E, currently in the Feb-Aug 2019 timeframe.
So not only does the RN need jets, but it needs a training cadre to work with US training and operational units to get started. It's easy to see where an organically supported UK carrier strike force (that is, with UK-based F-35 crew training) would not be up until 2023.
Conversely, the Marines' plan to declare IOC before IOT&E, with Block 2 S/W, could help the RN get started earlier.
On the other hand, this does mean going to sea trials in 2017 with a Block 3 aircraft, with new processors, that hasn't technically been cleared for US operational use. Chinook HC3 anybody?