River Mersey

The Mersey was a place in Liverpool, adjacent to Liverpool Docks.

When explaining to the Thirteenth Doctor how he was aboard a Sontaran flagship, Dan Lewis replied that they were building spaceships "all along the Mersey". (TV: War of the Sontarans)

Behind the scenes
Although not made explicit in War of the Sontarans, the River Mersey is Liverpool's main river which runs along Liverpool's waterfront, including Pier Head. The River Mersey can be seen in both Welcome to the TARDIS... and The Halloween Apocalypse.