British Defence Staff in the United States
Based at the British Embassy in Washington DC, the British Defence Staff deal with all aspects of US-UK defence matters. Site provides a range of information on UK Defence for an American audience.
http://www.bdsw.org

Defence Academy of the United Kingdom
Aims to provide civilian and military personnel with a high-quality education, primarily at post-graduate level, and conduct research in fields related to defence.
http://www.defenceacademy.mod.uk