I know, in the online sense, a dozen or so playwrights and screenwriters in London, some very active in various online writing communities like this one.
Look harder.
If it's plays you're more interested in, places where theater/theatre is big--London high on the list--are certain to have writing and critique groups. Call the dramaturge at various theatres, from professional to rep to amateur, and see what they can recommend.
Maryn, occasional playwright, frequent lazybones