The argument is initially flawed... It should be broken down to get a better idea of things.
The questions you need to ask are:
Do we need a Longe Range Fixed Wing Land-Based ASW aircraft?
Do we need a L.R.F.W.L-B.ASUW aircraft?
and
Do we need a L.R.F.W.L-B.SAR aircraft?
I know Nimrod did a whole lot more, but these 3 tasks effectively cover everything you could want from a LRMPA.
First of all, ASW. Ask anyone at Kinloss what asset the MoD needs to hunt submarines, and the answer will be an aircraft. Ask the same question at Faslane, and you'll be told that Sub vs Sub is the best scenario. I'm sure if you asked a Frigates crew, then the answer would be supporting their capability.
The obvious answer is that you need a layered defence, but you could argue the 'Nimrods' job can be done by SOSUS/IUSS (details available on open source), and as such the 'Long Range' element for UK defence is done by these systems.
ASUW, it's another argument against the need for a large aircraft such as the P-8 or Nimrod. A Dash-8 or King Air 350ER could do an excellent job with simple kit (probably still more capable than the MR2 or P-3), and still give around 10-12 hours endurance.
Finally SAR. Do we need a Long Range Search and RESCUE aircraft, or would a Search and LOCATE aircraft fulfill the required role. It is unlikely that this argument would every be finalised with an answer, but in theory the C-130 or A400 could do a good enough job to cover this role. Or if the rescue element is not needed then your ASUW aircraft comes into the fore.
I'd love to see an indiginous maritime patrol aircraft, but thats not going to happen. Does the UK need a single platform that can do all of the roles above? Arguably no, arguably yes. Those roles must be filled and the government hasn't denied that, but they are being done by a half dozen different assets (at least). Would there actually be any benefit from a single platform fulfilling many roles? Or is it better to have simpler platforms filling a single role?
Devels advocate on that one.