So I'm traveling to the UK for the first time, any advice UK redditors may have for me?

I posted this in /r/AskReddit but was told I may get a better response here. This is my first time traveling to the UK, London specifically, any advice on how to best interact with people so I don't stand out too horribly and possibly wind up offending someone? Any hidden sites to see instead of the usual tourist sites? This is also my first time traveling abroad so if anyone has lots of experience with that it'd be really helpful to pass it along.