SASless although written in the wake of the UK government's Strategic Defence and Security Review of 2010 (for which read desparate money saving measures) this commentary written for RUSI in 2011 addresses the problems that were likely to arise in the 'capability gap' between the cancellation and destruction of Nimrod MR4 and the arrival of the UK's P-8s. It also assumes a Merlin HM fleet of 40 not 30 (less the ones fitted for the ASACS role at any particular time).
https://www.rusi.org/explore-our-res...are-capability
Anti-Submarine Warfare: A Layered Capability
Developing an effective ASW capability requires the inter-weaving of several layers of capability. Each layer fulfils a particular function, making a particular contribution to the overall effectiveness of the ASW web. Yet, with the submarine threat to the UK much reduced since the Cold War and with the UK now focused elsewhere, some argue that the ASW requirement has also reduced. Certainly, the UK's ASW capability has dwindled in terms of significance as well as systems and skills sets.
The UK's ASW layers have thus been steadily eroded. The Nimrod decision, however, has highlighted quite starkly and suddenly the consequences of this. Despite MoD statements that Nimrod's roles will be covered by other assets, no other assets deliver its specific capabilities. The UK's ASW web hence has a particular, and significant, hole in it.