Since London is a dual port, connecting to both North Sea and North Atlantic zones. Can Germans do convoy intercept if they control the North Sea with a fleet on British convoys out of London that use the North Atlantic zone for convoy to France? Essentially, does a dual port count as a space in both naval zones and thus available to convoy intercept if that port space is used?
It does allow the use of both, but having a dual-port is an advantage, not a disadvantage. It allows a choice between routes. So - no.
This means that the Axis player(s) must grab a port in the NA before they can really open up a viable strategic naval campaign against the UK/US.