We've had immense success at the Treasury in cutting slashing and lopping of chunks of the Armed Forces ever since
HMG has actually invested heavily since the end of the Cold War in areas that are important in countering asymmetric threats that we now face - SIS, MI5, GCHQ, and UKSF have all had increasing annual budgets since about 1995.
We may have a fewer aircraft now in proportion to our European allies compared to twenty years ago - but that is because they have all been very slow and inefficient in readjusting spending priorities compared to UK.