For whatever reason Blizzard tries to guess location of the user based on the browser’s language preferences (while many other sites try to guess users preferred language by their location).
If you set en-gb (british english) as your preferred language in your browser it should direct automatically to en-gb site. In Firefox this is done by opening Options -> Browse down until Language, click “Choose” button next to “Choose your preferred language for displaying pages” and move en-gb to top. Add it if it’s not present.
Note that it can also affect sites which use the feature properly (not that many) so some sites might default to English instead of eg. your local language.