Even if this was not conscious bluffing on Macmillan's part, it was an empty threat. A "'fortress Britain" policy was just not feasible, given the opposition of both the other non-Six OEEC states and the United States:
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"Since the negotiations in the Maudling Committee had stalled in the spring of 1958, there had been talk among the outer Seven – particularly in Britain, Norway and Sweden – of possible retaliatory trade measures of the non-Six OEEC states, should France finally veto the FTA. When visiting London in March 1958, for example, the Norwegian Trade Minister Arne Skaug was already “breathing fire and slaughter”, according to Norwegian officials, because of French obstruction of the FTA negotiations and gave the British the impression that “he was anxious to propose new alignments between the United Kingdom and the Scandinavian countries should the Free Trade Area fail”. 14
"Macmillan, too, was extremely concerned about possible British exclusion from a common market of the Six and frequently referred to the possibility of retaliatory measures in a West European trade war. In June 1958 Macmillan wrote in an internal memorandum for Lloyd and the Chancellor of the Exchequer, Derek Heathcoat-Amory:
”If little Europe is formed without a parallel development of a Free Trade Area we shall have to reconsider the whole of our political and economic attitude towards Europe. I doubt if we could remain in NATO. We should certainly put on highly protective tariffs and quotas to counteract what little Europe was doing to us. In other words, we should not allow ourselves to be destroyed little by little. We would fight back with every weapon in our armoury. We would take our troops out of Europe. We would withdraw from NATO. We would adopt a policy of isolationism.” 15
"Three days later Macmillan told de Gaulle during their first encounter in Paris that “if we were to be threatened by a trade war by the Six we would be driven back on ourselves and would have to seek our friends elsewhere. (...) [It] might even spell the end of NATO.” 16 And in October Macmillan threatened the German Chancellor Konrad Adenauer in a personal letter that “we shall have to retaliate (...). The real danger is for the political unity of NATO. The collapse of the Free Trade Area could lead to the break-up of NATO and of our defensive system (...).” 17
"While Macmillan was still contemplating a “fortress Britain” policy in October 1958, 18 however, the political and administrative elites of the outer Seven realized clearly that – particularly in view of strong United States support for the EEC – their political position was much too weak to start a trade war with the Six. Nor was it in their economic interest. The Deputy Secretary of State in the British Foreign Office, Paul Gore-Booth, noted laconically on the margins of one of Macmillan's internal trade war memoranda:
“A nation of shop-keepers living on international trade and finance and importing 50% of its food-stuffs cannot turn itself into a self-supporting fortress except possibly at a drastically reduced (and electorally unsaleable) standard of living.” 19
"Essentially, the same was true of all of the outer Seven, and particularly of the industrial export nations Sweden and Switzerland. Thus, after a short period of recriminations over the final breakdown of the FTA negotiations the outer Seven in the first instance concentrated on alleviating the adverse effects of discrimination in the EEC market as of 1 January 1959..."
Wolfram Kaiser, "Challenge to the Community: The Creation, Crisis and Consolidation of the European Free Trade Association, 1958-72"
Journal of European Integration History 3 (1997)
http://www.eu-historians.eu/uploads/Dateien/jeih-5-1997_1.pdf