I think if you are concerned over the commonality between the A & B you are missing the point.
If we choose to add the A to the mix we will find it has a near 100% commonality with all the other F-35As in the global fleet. Same is true of the F-35Bs, albeit with a much smaller global fleet size.
With the F-35 we will not be guessing on a UK spares buy to cover our presumed usage (before slashing it in a future mythical savings round) and trying to keep the aircraft going as a UK cottage-industry, with robbing as our usual spares bank. Spares will be governed by the false-God that is ALIS and unique fleets or fleets-within-a-fleet will not be the allowable norm. Think UK C-17 rather than UK E-3.