I think people may be getting a little carried away. Labour and the Tories were both committed to 2% of GDP going in to the election and while a future Corbyn defence review might change things drastically, as might an economic collapse, I'm not sure it's necessary at this stage to plan quite so enthusiastically for something that hasn't happened yet and may not happen.
The article linked at the head of this thread made clear that the main squeeze is in the next year or so - which suggests finding a few more "efficiencies", or re-phasing a delivery or two, might be enough to sort matters.
I remember before SDSR 2015 some on here were similarly queuing up to prophesy swingeing cuts then too, and announcing that there was certainly no money for things like P8...