First off, it was a large trading bloc. Though the British Empire was not a customs Union, still less a true single market, the imperial preference became important as Britain abandoned free trade as a result of the First World War. In the trade wars of the thirties trade within the Empire and Commonwealth became even more important with imports from within the bloc rising to over 40%. When that bloc collapsed the safe harbor that Britain immediately set sail for sail for was the then Common Market. And 1940 Britain did not “stand alone” - the war against Nazi Germany depended on Canadian, Indian and African troops. It is a dangerous illusion that Britain can ignore what happens on mainland Europe. If Putin succeeds in his aim to prize apart NATO and the European Union, in the future Britain and faces a hostile power across the channel , Britain will be truly alone in a way that it never was in 1940. Even the USA, in 1940, will be in grip of Putin’s ally Trump.
It is a dangerous illusion to think that after Brexit, Britain can go it alone, because we never really have.