Crew costs are huge in New York and since moving houses is labor intensive and it is often triple time, the cost is prohibitive.
I am not sure if this is true of the West End, but many Broadway shows are so over-designed that a move cannot be made all that quickly.
Long gone are the days when a multi-house switch can be achieved in 48 hours. 40 years ago My Fair Lady closed and loaded out of the Broadway, I Can Get It For You Wholesale moved from the Shubert to the Broadway and Stop The World, I Want To Get Off came from out of town and loaded into the Shubert. All of this happened between curtain down Saturday night to curtain up Monday night.
"If my life weren't funny, it would just be true. And that would be unacceptable."
--Carrie Fisher