Given Watkin's record for thinking big, I don't think it is possible to say he never intended to connect to France. Also the point on the 'continental' loading gauge is relevant - Berne Gauge wasn't agreed and adopted until well after the GCR was constructed, so how it compares to contemporary Chemin de Fer de Nord loading gauge I don't know but would be interesting.
Was the GC not interested or simply too late? Not only was most of the traffic already spoken for, but the shareholders of the other major lines would have had considerable weight in parliament to prevent competing lines and services being started. As
@edwin_m mentions, the Met may well have intended to provide a cross-london Link not unlike that which Snow Hill tunnel offered, but just became a victim of it's own success.