You are pretty safe to bet that the long-running musicals will be here when you come to London. Shows such as Les Miserables (the world’s longest-running show!) continue to play in the West End, alongside shows such as The Lion King and The Phantom of the Opera. Plays tend to have shorter runs in London although thrillers The Mousetrap (the world’s longest-running play) and The Woman in Black are mainstays of the West End.
If there are shows you are desperate to see it is always better to book in advance.