It was set up to foster relations between English speaking people. They don't speak about it on their website, but I am convinced it attracts people who have an interest in maintaining the "Special Relationship" between the U.S. and the U.K.
Here is just something from the internet (wikipedia):
he English-Speaking Union (ESU) is an international educational charity which was founded by the journalist Sir Evelyn Wrench in 1918. The ESU aims to "bring(s) together and empower(s) people of different languages and cultures," by building skills and confidence in communication, such that individuals realize their potential. With almost 40 branches in the United Kingdom and over 50 international branches in countries around the world, the ESU promotes a variety of activities such as debating, public speaking and student exchange programmes, runs conferences and seminars, and offers scholarships, to encourage the effective use of the English language around the globe.
The mission statement of the English-Speaking Union (as stated on its website) is to:
- Provide a forum for international friendship through the support of the worldwide network of ESUs and the provision of secretariat facilities for the International Council at the ESU's headquarters at Dartmouth House, London;
- Facilitate and assist the establishment and recognition of ESUs worldwide;
- Focus on key current affairs issues through regular international conferences, seminars and taff and voluntary helpers;
- Work in close and innovative partnership with its corporate members and sponsors.