The few dozen ASW (anti-submarine warfare) destroyers and frigates in European navies are not going to suffice to protect European overseas shipping against a blockade or disruption campaign that uses many submarines. The U.S.Navy has more ASW warships (and SSNs), but any claims that it could defend American maritime trade against a fleet of SSIs (conventional submarines with air-independent propulsion) is laughable. They are worse prepared for that job than in 1941.
The ratio of escorts to cargo ships, ports and routes is simply too bad. Many if not most ASW ships would furthermore be needed as escorts in naval actions (same as the USN's problem in 1942) and many if not most would likely be revealed as ineffective once put to the test of a real war.
ASW aircraft, the handful of SSNs and the handful of (slow) SSIs aren't going to change the general picture of insufficient resources IF there was a major challenge by undersea forces.
I wrote back in 2009 about the (in my opinion actual) European naval power needs, and concluded that an offensive strategy to securing maritime trade would be successful and wise economically. I stick to this opinion, but it's interesting to think about what would at least be feasible (affordable) as a defensive and effective European (ASW) effort.
Well, the ratios are undeniably bad, so one needs to work on those. A reduction of routes would help greatly, and a reduced emphasis on the expensive and slowly built ASW warships. Navies L_O_V_E ASW frigates et cetera because navies are focused on hulls, on officer career opportunities, on prestigious things to show off*. They are by their nature not much interested in what air forces can contribute to ASW, in fixed installations, in land-based assets, in mines et cetera. There were interesting American efforts during the Cold War such as SOSUS and CAPTOR, but I'd like to assert that particularly the European navies have become even more focused on their pet interests (hulls, officer slots, prestigious things...) than before they suffered from the "peace dividend".
Naturally, that's where room for improvement should be searched first: All that stuff that navies don't like all that much, maybe even fear instinctively.
So, how about this?
Defended corridors could be established** with***
(1) anchored floating radar decoys
(2) area surveillance sonars (multistatic very low frequency active sonars, both operated by tug ships and semi-stationary like SOSUS)
(3) land-based ASW helicopters with dipping sonar and lightweight torpedoes that verify possible contacts and if need be engage them
(4) land-based anti-submarine missiles (~ASW-SOW "Sea Lance", with in-flight impact point update), on call by the ASW helicopters and possibly used to deploy lightweight torpedoes in patterns to prevent the sub's escape by sprint
(5) mine countermeasures assets that create and recreate/maintain safe lanes close to harbours
(6) unmanned submersible inspection drones that verify possible contacts even if helicopters
(7) acoustic decoys employed by boats (to make over-the-horizon attacks with sub-launched missiles too inefficient)
I don't even suggest to use frigates for this because their sonar wouldn't offer anything on top of this and the same applies to their onboard helicopters.
The civilian ships moving through these corridors would at the very least possess a towed radar and sonar decoy, and torpedo fuze-triggering decoys against wakehoming torpedoes. They would also deploy chaff and multispectral clouds if they spot incoming missiles or get alerted at such a threat by radio.
Such defended corridors could be established as follows:
Rotterdam-London (Royal Netherlands Navy)
London-Le Havre (Royal Navy)
Le Havre-Bordeaux (French Marine Nationale)
Bordeaux-Lissabon (Armada Española)
Lissabon-Gibraltar (Marinha Portuguesa)
This would be akin to the different sectors defended by the different national contingents in Central Europe during the Cold War (both in regard to land forces and area air defences).
I'm not proposing a German-run corridor because a Hamburg-Rotterdam corridor would be rather long and expensive compared to the modest role of German North Sea ports compared to the huge port at Rotterdam (Hamburg has only about 1/3 the cargo throughput of Rotterdam) and Germany needs to provide more pressing defence efforts than ASW due to its relative vicinity to the Eastern NATO and EU frontier.
The Mediterranean could be sealed off at the Bosporus (Türk Deniz Kuvvetleri/Turkish naval forces effort) and at Gibraltar (Royal Navy and/or Armada Española effort), with the same or similar technologies as used in the defended corridors.
SSIs and SSNs could track their unfriendly counterparts in peacetime or times of crisis, ready to pounce once war breaks out.
_ _ _ _ _
Navies are not interested in much of this, of course. They want ships, officer career opportunities, prestigious objects etc. The current Western naval strategies are reliant on land-based offensive efforts against bases (good choice), tracking, maritime surveillance, some mine countermeasures and otherwise the Western navies are all about "interventions": Things such as a battalion of marines or two to be landed in some Third World country, some disaster relief, some naval blockades for embargo enforcement, some launching of ship/land cruise missiles and naval aviation. There's no real concept to secure maritime trade directly, unless one has a ridiculous amount of confidence in the (implausible) ability to seal off the GIUK gap.
*: Just look at how often they talk and write about "showing the flag", a usually perfectly benefit-free exercise.
**: Effective width (unless restrained by land masses) equal to 2.5 times the realistic fibre-optic torpedo guidance range.
***: Protection against armed merchantmen would additionally require a surveillance, tracking, identification and boarding capability. This could be done with over-the-horizon radar (surveillance, tracking) and helicopters (ID and boarding) or alternatively many fishing boats with a few marines onboard each substituting for the helicopters.
P.S.: I know that this is mostly a clarification of what I wrote in September 2015. The assignment of corridors to the countries is an in my opinion intriguing addition, though..