We had tickets to see the Abridged History of America at the Criterion Theatre. Given the events of September 11, this was a particularly poignant show for us to have chosen. We went there on September 18, and the theatre was quite empty. It is a comedy performed by three Americans. They also do the Abridged Works of Shakespeare on alternating days. It is amusing, but the Shakespeare is a scream. Maybe we were just not in the mood to be too amused by our American foibles.
What I really wanted to talk about was seeing shows in London in general. It is really quite easy and not nearly as expensive as in the States. We always order our theatre tickets ahead of time. There is a half-price ticket booth in Leicster Square, and if you have more time than money and no preference for a particular show, that is the way to go. For me, my time in London is much to precious to waste waiting in line. I order my tickets from the individual theatres and pick them up the night or afternoon of the performance. There is a great site at: Londontheatere.co.uk/index/shtml. This site will tell you what is playing, and you can search by name of show or by type, i.e. comedy, drama, musical. It will then give you the theatre name, address, and phone number and tell you which tube stop to take. Also at this site are reviews of the current show, updates on openings and closing, and seasonal specials.
They even have theatre seating plans, so you can decide which sections you prefer to sit in.
I have used it with great success. Make sure you plan this as far in advance as possible, since some of the popular shows sell out months in advance.