I remember the figure of 400 as well - don't know if that included UK weapons as well (e.g. the Chevaline upgrade to Polaris was mainly about defeating the ABM defenses around Moscow).

NB An interesting article by Charlie Stross on the relationship between the US and the UK's "independent" deterrent:

http://www.antipope.org/charlie/blog-static/2013/04/on-the-u...