DREES CODE

There is no dress code for theatres generally. On special occasions such as the Royal Variety Show, or an opening night for a new show, people often wear formal dress. If the event you are attending requires formal dress, you should be informed before purchasing your tickets.
Even with air conditioning, theatres can become very warm in the summer, and the larger venues can get quite cool in the winter. Dress comfortably, bearing in mind that you may be sitting in the same position for a long time! CruisesLunch cruises normally have no particular dress code, but evening dinner cruises will often require smart/casual dress (i.e. not jeans/trainers).
SportNormally there is no dress code for sport events. Special events such as Royal Ascot are the exception. Check before attending events of this nature. Different dress codes may apply to different admission areas in Ascot.
Concertsgenerally, concert venues have no dress code whatsoever. If the event is a Gala/charity performance, some of the audience may wear formal dress - this is normally optional, unless stated otherwise on the ticket.
Tours/Museums etcNo dress code normally applies to any museum or general tour in London.