HMS Gartside was an Upholder-class submarine deployed in the North Sea by the Royal Navy. Though originally retired by the Royal Navy, the Gartside was commissioned again for training purposes. It carried an armament of several torpedoes, along with a small submersible called a Bathyscaphe, which it used for maintenance at sea.
While cruising through the sea, the submarine came close to an underwater Anomaly, causing electrical interference. The submarine was then forced to surface and was chased by a Liopleurodon. The Pliosaur apparently gave up chase and later a Swimming Theropod ended up on the bridge.
The Navy's Admiral decided to call the Anomaly Research Centre and report this. Matt Anderson, Connor Temple and Abby Maitland came to deal with the incursion. The captain complained he was only permitted a minimal crew. The theropod was placed in a freezer to keep it in a shocked state and Connor created a torpedo that would lock the Anomaly. While sailing toward the Anomaly however, electrical interference caused the submarine to lose power and a Liopleurodon crippled the submarine's propeller. Drifting, the submarine was pulled through the Anomaly, by its magnetic force, into the Jurassic seas.
The theropod came loose and terrorized the submarine's crew. Connor tried to find a fuse boxes to restore power. They later managed to restore power, expel the theropod through the torpedo bay, pilot the submarine back to the present day, and lock the Anomaly using the Anomaly locking torpedo.
The Upholder-class of submarines was the last of the non-nuclear submarines used by the Royal Navy. Upholder-class submarines were decommissioned in the late 1990's, but recommissioned for training purposes in the late 2000's. In real life, there is no Upholder-class submarine that shares the name HMS Gartside. None of them have an external bay containing a bathyscaphe either.