Hi, my Kindergarten class did Flat Stanley project the past 2 years. I always asked people to fill out a questionaire about their city. But if they didn't send one to you, just tell them things about London that are different from where they live. What's the weather like, what kind of foods do you eat, maybe you want to tell them that people use words like "loo" instead of bathroom, things like that.
Write a note from Flat Stanley's perspective and tell about the things he did.
For pictures, just do the landmarks like Buckingham palace, tower of London, London eye, Big Ben, Parliament, etc. And very important--hold Flat Stanley so that he is in the pictures somehow. That away you really are telling about Flat Stanley's adventures!
Hope that helped!