goal of 138 UK F-35s was calculated on replacing GR4 as well as equipping two carriers for strike and air.
There will be others better qualified to comment but the original 138/150 had nothing to do with the GR4 aspiration. It was the through life requirement to deliver 4 x 12 (14?) a/c frontline sqns + an OCU for Carrier Strike. I think the original intent was 36 FE@R embarked in the CVF. The GR4 issue arose as manned FOAS started to die an early death.
As far as JSF workups and embarkations go I am sure we will follow the USN lead, including working up at FALLON etc so that the RN CAG can co-operate off the US CVNs in theatre.
With an aviator CinC and soon to be 1SL/CinC perhaps the RN may get more robust on this issue.