Naivety of the IPPR report
The IPPR report undoubtedly gives the impression of being a structured consideration of UK's defence priorities, based on foreign policy goals, linked to the likely threats, current resource constraints and so on. However is that all that drives UK defence policy?
If MOD took IPPR (and others') perspective, it would join several European countries, in having a very practical, cost-effective, but essentially 'reactive' military capability. The IPPR report ignores the arguments for maintaining UK's strategic and technological soveriegnty (it effectively argues to pool this in Europe).
Over the last 40+ years UK has invested many billions in technology which was very expensive, unprofitable, was never used in anger, but which did maintain UK's strategic advantage - such as nuclear warheads, intercontinental missiles, long-range bombers, submarines, aircraft carriers, battlefield weapons, jet fighters, air-air missiles. Many of these products could have been sourced overseas more cheaply, but they weren't, because once a nation pulls out of this technology, it is generally irreversible.
Therefore, despite the IPPR and others' 'disarming' arguments, the UK may well carry stretching its budget up to and beyond the elastic limit, to try to stay in the 'high-tech' end of military capability, however illogical and even perverse it may appear.