In a lot of video games you can choose to play as a warrior, a wizard, or an archer. Which one you choose will totally change the game. Everything that happens in the game is connected to this one decision. Pressure!
The decision of which 'character class' to choose is often the hardest part of the game. It's normal for gamers to spend hours and hours researching this decision before they finally make a choice - then they spend the rest of the game wondering if they did the right thing.
(By the way, I normally start as an archer so I can kill bad guys before they get close to me...)
This 'decision paralysis' is how students feel when it comes to learning British English or American English. "If I learn British English I can't get a job in America! I like American movies but the exam is from Cambridge! Argh! What should I do?!"
Or they have crazily fixed ideas about what they want. "I want a British teacher because that's the best English. I don't want the American teacher even though she's really funny and she's the best in the school. I want to learn British English and that's that."
Listen. There are some differences between British and American English, but they are so tiny it's barely worth talking about. Yes, you often see tables of words with the American version on one side and the British on the other. But there aren't any charts that show 99% of words which are the same on both sides of the Atlantic! I almost don't teach US vs UK in my classes now. What's the point worrying about it when you can't decide between present simple and present continuous?
My advice: do not try to learn 'British English' - learn English. Do not ask for an 'American teacher' - ask for the best teacher. Do not worry about British spelling - just spell words the same every time you write them.
Having said all that, there are some differences in usage. Americans don't often use the word 'fortnight', for example. And these differences are sort of good to know:
The last one shows how weird Britain can be. A British 'Public school' is what most people would call a 'private school'.
So in summary, know that there is a difference between British and American English, try to learn some of the key points, but don't spend more than 0.001% of your mental energy on the topic.