The Guardian has an article, titled "Spontaneous compulsion," about how improvised theater is the up-and-coming thing in British theater.
One interesting quote of local interest:
"In my theatre company Cartoon de Salvo's new show, Hard Hearted Hannah and Other Stories, we improvise a new play every night. To learn how, we travelled last summer to San Francisco, the world capital of "long-form impro" ("short-form" refers to jokes and sketches). In North America, where improvised theatre first developed from social work exercises with disadvantaged children, it is a minor but flourishing activity. Companies like True Fiction Magazine, 3 for All [my emphasis] and Chicago's TJ & Dave perform off-the-cuff dramas, tragedies and farces to sizeable crowds. As Tim Orr, a terrific improviser from 3 for All, [my emphasis] says, "More and more actors are saying, 'That impro stuff looks good.'"
The article also links to a blog entry by Brian Logan headed "Making it up as you go along is a victory in itself."
[UPDATE: Here are some more links regarding Cartoon de Salvo and "Hard Hearted Hannah and Other Stories": Cartoon de Salvo's website; the show's info at the website for The Lyric Hammersmith; Brian Logan's blog at The Guardian; thisislondon.co.uk review; Telegraph review; Guardian review; The Times review; and Time Out London review]