There will undoubtedly be some analysis done by Dstl to show the relative merits of different blends of A and B models, taking into account everything from maintenance and spares issues affecting a mixed fleet through to the differences in operational effectiveness due to basing options, air refuelling compatibility, combat radius and weapon load. I dare say it will be possible for the likes of Tobias Ellwood so draw some reasonably straightforward conclusions from all of that. But all that work (which the purely military perspective would like to think definitive) is likely to form only a small part of any decision. Presentational RAF v RN considerations will probably play in, but the UK’s greater industrial work share in the B is very likely to be its trump card with politicians. The aforementioned analysis would have to be very compelling in the A’s favour to overcome that.
Also, do you remember when the Germans bid up the number of Typhoons they’d buy ahead of the work share agreement, only to cut it afterwards? And all the bumping of gums that ensued in the UK? That’s going to be us on F35 in due course, I reckon. I can’t see how we possibly get to 138 while doing Tempest as well.