TL;DR: Get the Fujifilm XF 23mm f/2 R WR. It fits your budget and the weather sealing is essential for London. If you're heading to the UK, you definitely want something that wont die in the rain. I've been using the Fujifilm XF 23mm f/2 R WR for a long time and I'm totally satisfied with how it handles. It's built like a tank but still small enough to be discrete on the street. The 23mm focal length gives you a 35mm equivalent view, which is basically the standard for street photography. It's fast, reliable, and honestly, I've had no complaints about the image quality. It's a very safe bet for a beginner because it just works well every time. Plus, it wont block your optical viewfinder on the X-Pro3 like the bigger lenses do. Definitely the most sensible choice for 400 bucks.