Phil
Defence Strategic Guidance and the Defence Programme Directory (reissued every two years) are meant to bring the SDR-baseline up to date using scenarios that could occur in the future. Unfortunately SDR did not forsee 9/11 and the subsequent forays into IRAQ and AFG and so we were always on the back foot in terms of 'the balanced force'.
The RN love SDR because it justifies the carriers
. The Army love the "AFG only" as it benefits them. Many of the scenarios are (alledgedly!) written to justify various equipment programme lines. Any future SDR, if it is to be successful, must have no 'sacred cows' (and in that I mean such things as carriers, future Trident, GR4 replacement, FRES (if that still exists), A400, FSTA etc). But I wouldn't bet on it.