Buying only 12 airframes? That (due to it's different sized internal bay) need all the RAF weapons to be trialled and certified again? That, while there are some similarities with the F-35B, offer little spares or training commonality? That can't be refuelled by RAF assets?
To carry, what, 4 nukes? Maybe 8 if the F-35 serviceability Gods are playing nice? That's not a nuclear deterrent, its a desktop review attempt at something.
The UK already has a nuclear deterrent, this idea simply takes money and focus from the conventional forces for little benefit.
Wholly year late post Batman (sorry), but the real reasons are almost certainly different than the nuclear mission.
F-35A has markedly longer legs and endurance. It is significantly cheaper to acquire. It has less maintenance demands (STOVL system).
Re aircrew and maintenance training - it’s basically the same, and there are plenty of people in both disciplines who have cross trained across A/B/C variants. While there likelihood that some of the UK weapons will need clearances for the A - wing mounted it’s a non factor. Software wise it’s a non factor - the benefit of F-35 having common mission systems. So it’s only clearing something like meteor from the A model weapons bay, which
the Italians are doing anyway.
For the RAF, the A becomes compelling for the following reasons:
a) cheaper acquisition - you can buy 12 for the price of about 8-9 B’s.
B) legs to get to Europe/Baltics and back in a sortie.
C) more endurance - what does this mean for the OCU (where they are going to be primarily used)? More training reps per sortie/event.
D) more available…. More jets on the line every day.
With a) almost certainly being the main driver. But could the RAF/UKMOD just say this? Unlikely.