Well, Dance of the Vampires definitely didn't do a try-out
(though it bloody well should have). To answer your question
more specifically: wouldn't the WEST END show be the tryout?
What's the point of a try-out if the show's already successful?
Think about it - for some time :)
Who can explain it, who can tell you why?
Fools give you reasons, wise men never try
-South Pacific