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.
So you're saying that the UK already has an adequate maritime surveillance capability and only needs a long range ASW capability. If all you're doing is defending the islands, why is long range ASW so important?