Woops! sorry should have included that november sub. I'm assuming a September invasion but in most cases it makes no difference if it was in June (even though an invasion in June or July was impossible). The Germans used the Channel in the 1st World War and between the wars and had access to pre-war Admiralty charts - it seems likely they would have the necessary data. The Admiralty issued regular bulletins to neutral powers about navigational dangers and changes in the channel and the Germans were able to get hold of them. I'm not assuming there was a surprise, an invasion alert was issued September 7 and RN ships (including the subs I mentioned) were sent out as a result, I think it reasonable to say that the response to an invasion alert September 7 is what would happen if there was an invasion alert.