The significant British naval ship that was sunk by the German submarine U-47 in October 1939 was HMS Royal Oak. This event occurred at Scapa Flow in the Orkney Islands during World War II and resulted in the loss of over 800 crewmen. The sinking by U-47, led by Commander Günther Prien, was an early success for the German Kriegsmarine and significantly impacted British naval morale.

Here is an image of HMS Royal Oak:

For more detailed information, you can refer to the comprehensive article on Wikipedia.