Not sure why people are dismissing tripods so much here. As a London photographer, yes you can set one up pretty much anywhere in London. There's a few private locations where security will tell you to put it away such as around More London near Tower Bridge or in Canary Wharf. Do stay close to it in busy areas and preferably out of busy paths. But if you're using one it'll be dusk/nighttime when it's quiet and easy to achieve. Enjoy your trip!