Actually,  it is number 10 Downing Street rather than Gordon who is twittering, mainly via Twitterfeed – http://twitter.com/DowningStreet

Dig around and you can find several mainstream information providers who are twittering,  although they are usually just sending their RSS feeds to Twitter via Twitterfeed rather than being actively involved in the community.  Run a Twitter search on the BBC, Timesonline  and the FT.