This is true, but perhaps the RN crews currently with the USN F-18s could switch to the USMC and train with the new GR9 squadron that they're about to stand up - VTOL Harrier ops more relevant to B etc
No, because the RN are after experience in embarked multi role carrier ops focused on delivering carrier strike. USN F18 ops from a CVN are more in line with what the RN is looking to deliver in the future when compared with USMC AV8B ops from a LHD.
Also, a tour with the USMC doesn't guarantee embarked experience. The only benifit with going AV8B is VSTOL experience which we are all led to believe is a very straight forward affair in the JSF.
The RNs biggest challenge and main argument for being involved in JSF is that they need experienced aircrew to run the show on the carriers. The bigger, more complex the carrier and the more time they spend on board the better.