Bastardeux, exactly right.
Getting back into the MPA game is THE big decision of SDSR 15 (the nuclear deterrent piece is separate although naturally linked). There is no doubt that the military want it (by which I mean each individual service). It all comes down to money (which can always be found if necessary- irrespective of the state of national finances) which is ultimately controlled by elected officials who often have other considerations in their minds than what is needed.