I've been visiting GB for over 40 years. Primarily London but also" smaller " places such as Durham, Boscombe, Salisbury and Lincoln.
All you need to do when there is to respect the differences and not compare anything to the USA. Two different countries.
Once witnessed a 40 year man from Nebraska rant that he couldn't find a Snickers bar. Wouldn't try a Cadbury chocolate bar instead. Wanted his Snickers and made everyone in the shop aware of it. Clerk was more than polite in dealing with this person as the tourist proved stupid is as stupid does.
Things you will notice and should respect is the quiet on the tube trains, standing to the right on escalators, talking loud in museums isn't the norm, don't jay walk (something you'll realize can cost you your life if you do), crisps are potato chips, french fries are chips, tipping is not expected by your bar person, smoking isn't allowed in any building. If you find a queue (line) wait patiently, don't complain. If wish to use a restroom ask for the toilet or loo. You'll often find them downstairs in building basements.
The bottled water concept isn't as prevalent there. If you want unfizzy water, ask for still. Petrol is gas. If driving get used to roundabouts and understand kilometers.
I keep returning because London is always changing yet always the same. Exciting, innovative city.
I also love the train systems which takes you away from all the hustle and bustle out into the country.
As it appears you are traveling through out GB you'll need to get used to the various accents.
Don't fret. Enjoy.