What one "needs" and what one can afford are two different things. Obviously the UK "needs" an MPA with a robust ASW capability. It has none. Would an MSA be better than the current nothing.
Well Ken that is of course a decision for any future government and a Strategic Defence and Security Review. But to answer your question, there are other assets that can do maritime surveillance, so it's not "nothing". Therefore what is needed is something that can do long range ASW; back to that SDSR thing I guess.