1. Just because they speak English does not mean you'll understand them. On two separate occasions I stood next to someone for over five minutes trying to figure out what language they were speaking only to suddenly realize it was English. At a comedy gig with an audience of mostly foreigners, this one poor guy did his bit; and nobody laughed the entire time because we couldn't understand a word he was saying.
2. "Pants" in the UK means "underwear." When you come in from the rain exclaiming how wet your pants are, awkwardness will ensue and people will wonder what exactly you've been up to.
3. Primark is your new best friend. Your nice boots that were supposed to last you ten years only held up a month in the Scottish weather? You're depressed about your limited wardrobe and need some brightly colored cardigans? Primark is where you can get a dress, sandals, cardigan, and sweater for less than $20. It's also a great place to go when you're in Cambridge and it won't stop hailing.
4. If there is a single ray of sunshine and you are inside, you're doing it wrong. People flock to parks whenever the clouds disappear, even if it's still only 40 degrees outside. On that note, you can't get surburned in 54 degree weather, right? WRONG.
5. Embrace the drizzle. Learn to love it. The secret to this is wearing too many layers; when you get really hot walking around you'll start to appreciate the constant refreshing mist on your face. Also, using an umbrella for drizzle is like having a "foreigner" stamp on your forehead.