In the West End, the Accent Is Decidedly American: . With Madonna, Gwyneth Paltrow and Matt Damon joining a throng of American performers in the West End, Kenneth Lonergan and Stephen Adly Guirgis among the dramatists getting rave reviews from the London critics, and world premieres from Neil LaBute and Christopher Shinn at top addresses, what are we to call the traditional center of the British theater? A blend of Broadway-on-Thames and Tinseltown U.K., perhaps.