Umbrella
Be sure to bring an umbrella or another form of rain-gear. London is famous for its dreary, drizzly weather and the reality closely matches the legend. While you'll likely get plenty of nice weather, calculate a few days of dampness into any London trip.
UK Plug Adapter
If you wish to use any electronics in London then you will need a plug adapter. Unfortunately, the plugs in England are different from those on the European mainland, so you'll need a UK-specific device.
Dressing Up
Bring one nice outfit. Whether hose and a skirt for ladies, or slacks and a blazer for men, London is a city that occasionally requires a bit of upscaling. Attending the theater or visiting a nice restaurant along the Thames will be enhanced by a little more formal dress.
Architecture Book
While a standard travel book will do a good job of pointing out items of historical interest, a guide to London architecture is even more likely to enrich your visit. So much history is stacked into every block of London that the standard tourist guides can only provide you a brief overview of data. However, much of the city's history is intimately intertwined with its buildings, statues and bridges, making an architectural guide a revealing document to have with you.
Tube Map
London has one of the most comprehensive (and oldest) subway systems in the world. Known as The Tube, London's metro system can be intimidating for first time users. Having a map with you can allow you to study it at your leisure, rather than being at the mercy of posted maps inside the stations.
