As far as I can tell in rereading the article several times, Buchanan is making no claim that free trade was wholly responsible for Britain's decline. Buchanan does argue that countries fared much much better under the cover of protective tariffs, which Professor Rummel does not really address in his reply. This is not a case of misplaced causation.
Reduced to single sentence, Buchanan's point is: "As China demonstrates, it is a mistake to assume free trade, or even democracy, is indispensable to growth."
And, as Buchanan claims and history confirms, while the United States was ". . . expanding westward, consolidating a huge continent, and integrating the resources and capability that GB could only envy, without any threat abroad or significant colonies prior to 1900 to drain these resources," its remarkable growth was achieved under the umbrella of protective policies and tariffs, not free trade. Wonder what the Republicans of that era would say about the free traders of today?
You missed the point. If GB hadn't had the wars at the time , they would have dominated the USA economically