I'm not underestimating Brit traditions here, but imho it's rather a matter of being a 'sunday boater' rather than a proper mariner - regardless of the passport and the sea.
Re. regulations, isn't 'aid omission' (generally speaking, not only at sea) a criminal offense?
Now, if/how/when refusing the entrance in a marina can be qualified as aid omission, that's another matter.
But I suspect that under a declared emergency, and if those refusing the aid could not prove to have compelling reasons to do so, they do risk to be prosecuted.