S-boats in the Thames Estuary

Could German S-boats reach the Thames Estuary to attack coal ships near London in late 1939 or early 1940?

They could reach the estuary and attack, but narrow waters, coastal guns, anti-aircraft weapons, and nearby destroyers made the return dangerous.

Reaching the Thames Estuary was possible, but surviving the attack and return was the harder problem. Narrow approaches exposed fast craft to short-range naval guns, anti-aircraft weapons, and any destroyers in the area. Chatham Dockyard and the Medway added nearby naval forces to an already heavily defended approach to London. An S-boat might get in and strike, but losses were likely. German raiders had good reason to choose targets farther up the east coast where defenses were thinner. Destroyers nearby could also use dual-purpose guns to help defend cities against air attack. The discussion connects that assistance with later local commemoration of ships, while leaving the examples uncertain.