The process will look like this:
1 - Identify threats to national interests (which include threats to international order)
2 - Define and cost responses to these threats that will preferably deter them and if necessary defeat them
3 - Prioritize threats
4 - From bottom to top, either eliminate or scale the responses until the budget target is met
Two overarching problems, not exclusive to UK:
1 - Limits on what can be done to some costs, such as personnel and overhead, which drive the cutting burden disproportionately to modernization (R&D/procurement)
2 - A very small number of very expensive programs (nukes, carriers) that are hard to cancel, limit or stretch (because the unit costs get even worse) but that mean that as the overall procurement budget goes down by X per cent, what is left for other needs goes down by XY per cent, because all the bills have to be paid by smaller programs.
GLWT...