What should one bring on a trip to London?
My answer to What should one bring on a trip to London? on Quora
Bring an unlocked smartphone (Google Maps is essential, it has excellent public transport directions—don’t trust Apple Maps). You can get a pay-as-you-go SIM when you arrive, provided your phone is unlocked (check that you will be able to put a UK SIM in it).
Having a good map on your phone will make a huge difference to your trip. London is a fantastic city to walk around, but it’s also easy to get lost if you don’t have a map. It might be worth grabbing a pocket sized London A-Z map from a book shop too.
Don’t get too hooked on the tube—for journeys of only one or two stops it usually makes more sense to walk, and longer trips are still more fun walking because you are certain to see lots of interesting things en route... and having a smartphone with data will let you find out what they are!
Carrying an iPhone completely changed my relationship with London.