Basically, what the UK is doing is rebuilding the GIUK gap barrier capability of the Cold War by using current technology of mobile systems instead of the fixed sonar arrays of lore. It's a bit like a more extensive group of the old SURTASS ships (SURface Towed Array Sonar System ships, they were the original "stealth" ships that the US could deploy. They dragged huge towed array sonar with much greater range of capabilities than the warship deployed ones and could be sent out far away from the convoys to provide early warning. They carried no ASW weapons.)