As an experienced LRMP pilot (four types with two Air Forces) my serious opinion is that two engines are not enough. Check the history of the Breguet Atlantique.
I can almost accept that 4 engines is better than two for an MPA due to the low level flying, though I would still disagree on the grounds that modern engines nowadays are so much more reliable than Speys or turbo-props.
There will be plenty of cheap, low hours, second-hand, big donk Airbus 340s on the market soon. How about looking at modifying them for LRMP? Most of the ASW kit has already been formulated for the Nimrod MR4 and most of the work could be carried out in the UK.
You cannot be serious when you are suggesting an A340 as a replacement MPA?
For all the reasons and hundreds of posts above regarding a UK "fit your own kit" MPA, never mind the fact that an A340 is way too big and expensive to operate, this is one of the daftest ideas I have seen yet.
If the UK buy the P-8A, that will be the best defence decision made in UK for many years.
Y_G