I'm not sure of the exact year, but it looks like it would have occurred around 1820-1830. The United States was just breaching 10 million near the head of that time period and so was England, which makes up the bulk of the population of the UK. After that point, the United States shot up and easily overtook the population of England.
England did not make up the "bulk of the population" of the UK in 1820. The UK then included Ireland, and that had more inhabitants then than it does now. Scotland's relative weight would also have been a good bit larger.
