Before going to London, I did some research on English schools in London and chose Oxford House College, mainly because it had reasonable fees. Also, there weren't many Japanese students at Oxford House.
I took my parents' advice and requested that my homestay family have both a mother and a father, be native-born, non-smoking, middle-class British people, and live near a subway station. I later found that this was very good advice, since some of my friends at the English school were having problems with their host families.