This was one of the best parts of my trip to London back in May. If you’re in Kings Cross/Exmouth Market area (heck, even if you aren’t!), you should make a trip and take the Mail Rail. Your ticket also gets you into the main part of the Postal Museum, which is great, too! There’s a ton of history displayed in really digestible exhibits. When I was there, there was even one about mail extracted from a well-preserved shipwreck!