http://www.whatsonstage.co.uk
Look up plays, book and pay your tickets before leaving. Couldn't get any better!
A good advice is to tick the box saying "I'll pick up my tickets at the box office" because you certainly doesn't want to loose a 40 pound ticket in the mail... I've only come across one theatre who didn't have that option, The Globe, but fortunately the 5 pound ticket found my mailbox anyway. :-)Close shave between experiencing Judi Dench and Kenneth Branagh on stage. But while "All's well that ends well" (Dench) was a sleeping pill during the first act "Edmond" (Branagh) was a one hour long hard punch in solar plexus. Would be my fondest theatre memory - so far.

