As explained by the RAF, the As are for training which saves a lot of money over using Bs for this. The NATO nuclear mission is a side mission. They're not going to be sat on nuclear alert all their lives.
Not buying As for this means spending more money buying more Bs, or using existing Bs and having fewer operational Bs.
Personally I'm not convinced that having a separate fleet of 12 A's, along with a fleet of 62 F-35B's would, after the additional support costs, disparate fleet etc, save any money at all in the long term....particularly when you factor in having to purchase, maintain and support US weapons to equip the F-35A fleet (we're not going to qualify Asraam or PWIV on F-35A for a fleet of 12 aircraft). I'd love to see the RAF's figures that they've hoodwinked Ministers with.....
The ONLY way an F-35A purchase of 12 a/c makes even the most marginal sense (along with everything else) is if it was part of a cunning UK MoD plot to get UK weapons integrated on F-35A to increase their sales....(Meteor appears to be going on F-35A regardless, but PWIV, Spear variants and Asraam are not unless someone shells out...and there are countries, Italy for example, who would be interested in some of those).
Even the nuclear mission makes zero sense now....the F-35A won't arrive and be operational until 2033/34 at the earliest. And thats just flying for training...NOT the nuclear mission. Training and establishment for a nuclear mission is not a simple process...and at the end of it we'd still be dropping US owned and controlled, freefall B-61/12....a choice that looks more moronic and unrealistic by the day...
If we really wanted (and I'm not convinced the present government actually does) a tactical nuclear capability we could, in terms of the timeline, actually put an Astraea warhead in a freefall casing, or better still a missile, and deploy on a far more capable and longer ranged, Sovereign platform in the form of GCAP in a not too dissimilar timeline....