Pontius,
LRMP are not there to find and attack SSBNs, they patrol a long long way away in a place that makes them safe from attack by said aircraft. LRMP can help to defend our own SSBN or they can help to stop somebody sinking our very expensive carriers when we try to "project" UK PLC.
The point has already been made; a large aircraft has the inherent flexibility required to adapt to different roles. It might be over Afghanistan now but where will it be in 5 years time? Can we afford to buy lots of specific types and then throw them away in a few years? If we are on the subject of expensive single-role aircraft then why not discuss Typhoon? Does Terry T have a hidden fleet of high-performance fighters that the Typhoon is ready to engage?
Get away from this "There is no submarine threat" Yes there is, if those people decide to use them! How much will oil cost if Iran decides to use those Kilos of theirs?
As to P8s and Boeing UAVs, take a look at a recent attempt to produce a large multi-purpose system. It was called the P7 and it failed dismally. UAVs for ASW? Try it with such a small FOV; needles in haystacks and then some! We break UAVs when we try to land them in tricky conditions; try flying one at low level whilst dropping sonobuoys and running for MAD. And before you tell me that the P8 will not have MAD that is because it was cut for cost, not because it was not required. That is why we still need manned LRMP.