0係 london 睇歌劇應該著咩衫?

If you are talking about Royal Opera House, then No, there is no dress code. But you have to checkin your any bags (including backpacks) into the cloakroom.

If you talking about Musical Theatre like Theatre Royal Haymarket and Her Majesty's Theatre, there is not much rules except not admitting babies or toddlers.

If you are talking about Royal Albert Hall, then, probably no, unless the event is specifically requested. You will know that by checking out the ticket and it should be printed on it.