Royal Navy submarines inside the Baltic

Did the Royal Navy raid merchant shipping in the Baltic, and did the Danish belts stop its submarines?

British submarines did enter and attack Baltic traffic, especially in World War I, despite the difficult Danish approaches.

The Danish belts constrained access but did not make British operations impossible. The Royal Navy sent submarines into the Baltic during the First World War to attack maritime traffic. This remains one of the more frequently forgotten British submarine campaigns. Some British boats also managed to enter the Baltic during the Second World War, although the First World War effort was the more substantial case under discussion.