I'm going to England in a few weeks, any international traveling tips?

More specifically around the Bristol area to visit my girlfriend, it will be my first time leaving the country so I’m a bit paranoid about customs and going through security and all that with my tech, and just customs in general. I’m not all that sure how exchanging currency goes but I heard something about it being more of a hassle, but a better exchange rate at a bank- any info on that?

I’ve googled ‘international flying tips’ and scoured the first few pages reading as much as I could before it all turned into ads, and I’m still a bit nervous/curious about the whole process. Are there any neat places I should be sure to visit over there? My girlfriend just moved there about two months ago so she hasn’t seen everything yet, which is awesome because I’d hate for her to feel the need to be my tour-guide (I’m going there with the intent to see her, the location is all just icing). I’ll be there for about a month.

She also expressed interest in taking me clubbing (something neither of us have done), but I’m not sure how that would work with IDs and such since I’d only have my passport and US ID, and I would prefer not to take my passport out of fear of losing it.

Oh; one more thing- I’m not too worried about the culture change but are there any no-no’s that aren’t too obvious?

General international travel advice is greatly appreciated too, even if you feel you’re just regurgitating stuff that’d be found from quick Google searches, hearing it in someone else’s wording can always do wonders for comprehension.
also any sweet romantic spots for two youngins (18/20) to visit would be A+ ;)