I must admit that I haven't yet found he time to read all of the link that kitbag provided. However, surely an SDSR in May was never going to be a player when the general election is the same month, and we are likely to get a government change?
A question I have asked on here before (but don't normally receive a response), is quite simply, is SDSR15 merely the intention/brainchild of the current government? As far as I am aware that is no rule/law that requires a UK government to hold an SDSR every 5 years. SDSR15 and FF2020 are merely plans put in place by the current government. For example, if UKIP were to form the next government, and decide to double defence spending, all staff work already done for an SDSR15 would be torn up, and the planning would have to start again.
My own personal prediction for the next UK parliament is for a minority Labour government, kept afloat on critical votes by minority parties, especially the SNP. Surely any new incoming government is going to want to get its feet under the table, tell MOD its defence policy (i.e. how much it's willing to spend) and only then can MOD respond with its plan - making September probably the earliest this could happen.