"Heathrow- I think you meant to say Canadians did under the BCATP not the Yanks. Some may have trained in the US but the vast majority trained in Canada."
I think there were 5 British Flying Training Schools in the US under the Arnold Scheme, producing a significant number of pilots, with PanAm training navigators under the Towers Scheme. Admittedly the numbers were not as large as those trained in Canada under the BCATP but they still provided a fair chunk of the RAF's aircrew in the Second World War.