If you mean at some point in history Britain had the world's largest exports I wouldn't dispute that but I also wouldn't understand the point you were making. The world was such a different place in those times and changing rapidly, even between the 1947 you mentioned and 1973 when we joined the EEC. The world economy of the immediate post-war period totally different to that 25 years later. I understood you to be making a reference to the period up to joining the EU given the discussion but fair enough, I'll happily admit at some point in the past the UK was the world's largest exporter.
Having said that, I very much doubt UK exports matched the US in 1947 though.