Perhaps a simple way to simulate an effective naval blockade against the UK would be for the Axis to block all direct sea routes to the UK with any naval units. That way there is no one space that becomes all important but instead forcing the Axis to actually blockade the UK from the rest of the world. So, if the UK can trace a sea lane from the US or its holdings in Asia or the Pacific then the Axis has failed to blockade the UK. Now as to the effect of such a blockade that is up to some speculation since who is to say what percentage of their IPC’s can be “blocked” in this fashion or are they just lost.
Perhaps you force them to seperate their income like in A&A Pacific. In that case you could say these “blocked” IPC’s be spent only on colonial builds by IC’s built hopefully in India, Australia, South Africa, or Canada. This could just complicate the whole thing which many ruin the fun and easy pace at which the game progresses though.
As to the original question I think that it is silly to have a nations Naval builds blocked by some naval units floating nearby.