The problem is, that all discussion regarding the loss of the MRA4 focusses first and foremost on SAR. A LRMPA is much more than that and SAR is but a small element of its role and indeed, if you look at the international requirements, the MOD legally, only has to legally provide for Military operations.
LRMPA is about (amongst other tasks) locating tracking and prosecuting submarines and other surface contacts. To do that you first of all need a high transit speed to get you to the location, then the ability to loiter for a long period whilst using a multitude of sensors, in all sea states to find, fix and track and then if required, deliver a weapon to remove any threat.
By focussing on Search and Rescue, the argument is over simplified and does not recognise the true requirement of blue water maritime patrol.