"must"?????
that's the sort of thinking that's got us where we are today
We need a PATROL Aircraft to start with - not an ASW platform . It would be nice indeed if we could afford one but it's clear we can't (or won't) fund a proper Nimrod replacement
According to the current "World Naval Review" the threat from submarines is NOT the same as it was in the '70's & '80's for example - Russia has only 1 post 2000 SSBN in service as of mid 2012 and a ragbag mix of 1970's and 1980's boats
Their surface fleet comprises around 22 warships of Frigate size and above - about the same as the RN - but without the maintenance plus its a 4 sea operation