Not many countries speak only English. This is because of the increase in immigration of people from different parts of the world. Over time, their local dialect gets into the system.
Most of the countries that are only English speaking were colonized by the British. Examples are: Antigua and Barbuda, The Bahamas and Barbados. Australia is also majorly an English speaking country.
English is spoken in many countries but most of these countries have other languages spoken by the people.
There are very few countries in the world who only speak English. There are certainly many who the majority of the time speak English but still choose to converse in their native languages. A good example of this is the United Kingdom territoral islands such as Antigua and Barbados. English is mainly spoken because of the huge presence of tourism. These islands also have native peoples who trace their roots back to the mainland of Africa.
Still, these people have to speak English in order for them to be able to succeed in anything that they try. Speaking only one language in a country would also make it one that a lot of people would call dull. The embracing of migrants and immigration so many years ago has actually allowed for the growth and the preserverance of the English language. For certain, English will always be a driving force in the optimal functioning of the world.