Here is a list off useful links for theatre:

London Theatre
This guide was created in August 1995 to provide a reliable source of information pertaining to London Theatre on the Internet. The objective of the guide is to be as comprehensive as possible for West End theatre listings.
www.londontheatre.co.uk

Official London Theatre
You'll find everything you need to know about London theatre - all the news, what to see and how to book - here at the official site of the Society of London Theatre.
www.officiallondontheatre.co.uk

UK Theatre Web
UK Theatre Web has an enormous database of information on people, plays, venues, performances, web sites etc. Plus gossip, editorial and related material to help you enjoy theatre, concerts and film in the UK.If you are researching information on plays, operas, tours, productions etc then see our extensive Archive.
www.uktw.co.uk