IMHO it shot it's self in the foot in SDSR 2010 by keeping too many FJ Sqns and taking too many hits in other areas (I know we lost a fair few FJs). I don't think the whole Force structure looked relevant for future Ops, I think this was proved by the subsequent loss of a couple more Sqns after SDSR as we still looked fat. For example many of our neighbors had already bitten the bullet and done this by halving (or more) their FJ fleets and making sure they were efficient and capable (Denmark, Norway, Netherlands, Belgium) also by mixing them with SAM systems, etc. while we persisted with the Battle of Britain concept.
Where to start with this??!
- The decisions that led to the 'loss of a couple more sqns after SDSR' - reducing the Tornado force from 7 to 5 to 3 squadrons - were all taken as part of that SDSR and its 3-month exercise, they just weren't announced or implemented immediately. But they were all laid out plainly in the classified plans alongside the headline chopping of the Harrier force.
- Keeping 'too many FJ sqns'? Within months of the SDSR, Op ELLAMY showed that we had cut them too far - the op was only possible without impact on Afghanistan because the sixth and seventh Tornado squadrons had not quite shuffled off the parade square yet. Then Op TURUS and Op SHADER forced the reversal of the PR11 decision to FURTHER cut the Tornado force to just 2 squadrons! And now I think pretty much everyone expects that the Typhoon force will grow to compensate for the retirement of Tornado, whereas previously we were going to decline yet again.
- 'Battle of Britain model'? Denmark, Belgium and the Netherlands all have the benefit of very small FIRs than can easily be protected by a combination of SAMs and QRA. Plus the latter two are well-surrounded by NATO members and will rarely be responsible for making the first intercept and identification of an unidentified contact. Conversely we have to defend an enormous patch, much of which is over sea, and quite often have to go and make first identifications, both things which SAMs are poor at. So there are very good operational reasons why we haven't copied the F-16/Patriot model.
The logic of SDSR10 and PR11 would have seen our fast jet force reduced from 30 squadrons during Gulf War 1 to just 6 squadrons in 2020. That isn't halving, as in the examples you quoted - it's an 80% reduction. Apart from those capabilities which have been deleted, I think you will struggle to find such a precipitous decline anywhere else in our forces. These cuts have been widely accepted as a mistake in senior circles and SDSR15 is almost certain to see the direction of travel reversed.
If I was looking for imbalances in our force structure, or questions of relevance to future ops, I would be asking what exactly we propose to do with 60 Chinook, a number which was only arrived at as a sop to an angry public during HERRICK. The size of that fleet is a legacy of a particular kind of war, the kind which Robert Gates advised any leader considering to "have his head examined".