Interestingly Google ngrams has 'poop' as more popular, even in British English. https://books.google.com/ngrams/graph?content=poo%2Cpoop&year_start=1800&year_

Interestingly Google ngrams has 'poop' as more popular, even in British English. https://books.google.com/ngrams/graph?content=poo%2Cpoop&year_start=1800&year_end=2000&corpus=18&smoothing=3&share=&direct_url=t1%3B%2Cpoo%3B%2Cc0%3B.t1%3B%2Cpoop%3B%2Cc0 https://books.google.com/ngrams/graph?content=poo%2Cpoop&yea...