This was actually with visiting, not living in the UK: I wish I'd known not to drink the water.
LOL it's such a cliche, but it's true: don't drink the water. Every single day, I was making protein shakes that I needed for my diet and didn't even think about how much tap water I was drinking. It resulted in something along these lines:
boyfriend *in a very VERY proper voice*: are you ill?
me: uh.... *covers mouth, starts throwing up*
boyfriend: *very shocked, absolutely hilarious look, still very proper* oh dear, we should get out of here....
me: *realizes that one kleenex isn't going to cover for this one, takes off trench coat, freezes ass off despite the fact that it's summer, stumbles back to the train station, gets annoyed that you have to PAY to use the bathroom here, rinses clothes off in sink, throws up again multiple more times on the train* Huh, so that's why I'm not supposed to drink the water. *spends the rest of the day in bed drinking ginger ale and sleeping. There's an expensive vacation well spent.*
That brings me to another point: paying to use public restrooms. I had a funny moment when I was standing outside the bathrooms in the Princes Street Mall ranting about "It's not that I can't figure out which one is a 20p coin, it's the principle of the whole thing!" and then a complete stranger hands me a coin to use the bathroom (or to get me to shut up).