Sorry SW,
I beg to differ.
We all know that the next 5 years is going to see the HM Treasury squeeze the MoD budgets beyond anything we have experienced thus far since the recession. Afghan has managed to be an artificial funding ring fence where we have managed to keep our military (less so the Fisheads) on a trained and operational footing.
The MoD has been able to provide HM Govt and the nation with flexibility/contingency by using those very same highly trained, well equipped and experienced personnel to deliver small bespoke 'shows of force' and support to the FCO/NGO (everything from small exercises in Poland/Estonia, assisting Air Campaign against IS to Ebola and Sierra Leone and helping with flood defences in support of the Environment Agency).
But Afghan is now a distant memory for both politicians and many of the public. There is no requirement to spend money on training many personnel to a high standard (as per the Afghan tour cycles). Those that did benefit from this training and the corporate memory of both technical, kinetic and yet simple war fighting operations are leaving in large numbers across all 3 Services of all branches and trades.
It appears that when talking about 'hard choices ahead' then many of the political elite/'leaders' either are taking the world's developing strategic geopolitical situation on risk, or recognise that they can only do small token operations on the world stage (therefore reducing any political leverage that may come with such a token force) or are ignoring the advice from CDS/PUS and independent think tanks like RUSI.
Therefore in answer to your question I personally believe that ahead of the next SDSR the next Chancellor of the Exchequer and HMT should provide the MoD with its 5 year budget, with SDSR providing and prioritising the Defence Roles it requires and then the MoD review and then tell the Government of the day what they will get for the budget that they have allocated.
If that means 'capability holidays' (rubbish term) or moth balling/suspended animation/long term storage of ships, aircraft, and infantry battalions then so be it.
Having a fantasy SDSR 15 that talks the talk about what the real threats are to our nation etc, that does not have the resourcing (notably finance, but also the right quality and experience of people) will only result in either increased overall financial cost, political risk in not being able to deliver and possibly worse of costing more lives.
I personally get the feeling that the politicians strategic vision is should the requirement arise for a strong, well equipped Armed Forces (Cold War really does return, or IS really does need sorting out or AQAP require military action or a different posture than our current token efforts, then the HMT will throw money at the problem (as per HERRICK after the first few tours didn't go to plan).
If any future SDSR is not funded or resourced it quickly becomes irrelevant and everyone is forced to make it up as they go along, fire fighting and reverting to the use of the term of 'best effort' which has seen us through the last 13 years.