Airpolice,
I'm not an expert on where HMG wants us to go next (expect the unexpected) but you don't just need F35 to go to war with Russia. Anything from Syria upwards would warrant F35 if you actually want some kind of freedom of mvr. In ten years time that kind of tech will be trickling to a lot of other places around the world and I'd rather have an F35 looking over my shoulder than a Gripen or souped up F15/18. (Be nice to have some Growlers along for the ride as well though, admittedly).