Seabed hydrophones existed before SOSUS-scale surveillance

Did Britain have anything like SOSUS at strategic places such as Gibraltar?

It had fixed seabed hydrophones in some locations, but nothing approaching the capability of the later SOSUS system.

Gibraltar illustrates the difference between early fixed listening devices and Cold War ocean surveillance. Britain had seabed hydrophones in various places and was moving toward a hydrophonic barrier concept. These installations could support local detection around strategic passages, but they lacked the coverage, integration, and analytical power associated with SOSUS. Britain and the United States later helped establish the Atlantic network together. A recognizable operational idea therefore preceded the technology and cooperation needed to make it a theatre-wide system.