Heathrow is on the right side of Europe for connections to/from the USA, and on the wrong side for connections to/from China (for Finnair in Helsinki the opposite applies, as is apparent from their own established route structure).
New Orleans does not have, I believe, any European flights at present whereas Chengdu, a place most in the UK have never heard of, has a surprising number of other nonstop European flights and operators to better connection points.
I think there would be a balanced 50-50 level of demand for the New Orleans route from both sides of the Atlantic, whereas Chengdu must generate little traffic originating in Europe.