Dialects of English
British English is supposed to be the form of English spoken in southeast England (and BBC TV channel) but it's understood in other parts of the United Kingdom. The section on pronunciation assumes the received pronunciation of British English, from which there are different regional variations. American English is the language spoken by U.S. government officials, the U.S. media, etc. It doesn't include Canadian English, that it's out of the definition of "American English". Although Canadian pronunciation is similar to that in the United States, spelling more often than not takes the Commonwealth form. American English is also adopted by many countries and organisations, like Japan, Liberia, South Korea, Taiwan, and the "Organization of American States", whose use of English is strongly influenced by the United States.
Commonwealth English refers to the language written in most of the English-speaking countries, including Australia, Hong Kong, New Zealand, South Africa and the United Kingdom. The language varies slightly from country to country or even between regions, states, provinces and territories, but it's always distinct from American English. Commonwealth English is mostly interchangeable with British English and where "Britons" is used, "inhabitants of the Commonwealth of Nations" might be more accurate. Commonwealth English is also used by countries and organisations, such as Ireland and the European Union, whose use of English is more influenced by the United Kingdom. International organizations like the United Nations, the IOC and the WTO also use Commonwealth English as a their standard language.