1. When in a restaurant, do not ask for the “check.” They will have no idea what you mean. Ask for the bill. (Side note about restaurants: the service here is nothing like in the States. They make a certain wage and don’t rely on tips, so the service is slower. You will have to ask for the bill because they won’t bring it to you otherwise. Also, expect to be given a single bill no matter what company you are in, as it is unheard of for places to split it. We have received many a stink eye for requesting to pay separately, so always be prepared. Many places will accommodate individuals paying their own part, just be aware of the cultural difference and don’t make a big deal out of being given one bill).
2. Queue: they don’t say “wait in line.” You queue (which is quite fun to say once you get used to it).
3. Cash Machine / Cash Point: very few people will know what you mean by “ATM” so be sure to say cash machine if you are looking for one.
4. Quid: just another way of saying pounds (their currency). So if someone says they spent 50 quid on something, they spent 50 pounds. Easy peasy. Also, they call their bills “notes” or “bank notes” but I haven’t encountered that as much.
5. You alright? This has to be one of the hardest phrases to adjust to for me. When you see someone, they will usually greet you with “you alright?” It’s just their way of saying hello and asking how it’s going. Every time I hear it I still think they are asking me what’s wrong or something, but it’s merely their version of “hey, what’s up?”
6. Food can be confusing. Fries = chips / chips = crisps / cookies = biscuits / dessert = pudding (though desserts is a common term).
7. Zed: the last letter of the alphabet in England is pronounced “zed” instead of the US’s version of simply “z”/”zee”. It can be really confusing hearing this for the first time. (And can you imagine calling Jay-Z “Jay-Zed”? It’s just not the same…)
8. Keen: this is possibly one of my favorite English phrases. When one asks if someone would like to do something and the other is up for it, instead of saying, “Yes, I’d love to do that,” they might say “keen!” Like they are keen / excited to do it.
9. Toilets: it’s a general rule not to ask where the bathrooms / restrooms are. They call them the toilets. (Honestly, I’ve only heard the term “loo” a couple of times, so I wouldn’t recommend saying that unless you heard otherwise in that instance).
10. Fancy: fancy is a very common term to say one “likes” something or someone. It’s not a measurement of style or posh-ness.
11. Academia is riddled with a ton of different words that I cannot even fully explain. “Professors” are considered the top tier of teaching in a university, so not all of the faculty are professors. Most are called lecturers or your academic tutors. Just note what your module handbook (syllabus) states about the title of the teacher.  
12. Higher education is called uni or university. They have primary school, then high school goes up to the age of around 16, and college is from 16-18. It’s their pre-uni schooling, so do not equate college with uni. Also, what they call public schools is what we call private schools, and vice versa. "Public schools" here generally mean exclusive and expensive. Very confusing.  
13. Yes, they do say mate when talking to / about a friend.
14. Finally, cheers. People use this as a parting term or a way to say goodbye. It took a long while for me to get used to hearing this one, but it is a very fun phrase once you adjust to it. 

I will say that one must be selective in the phrases they adopt. I mean that it is easy for people to come across as "phonies" for lack of a better word when they automatically assume the language of a new area. Use the words you have to in order to get by, but don't go around calling everyone mates and saying "cheers!" That's not what is expected of you. People will know you are not from the area and expect you to speak differently. I've encountered some American exchange students who started spelling things the British way and using all of the English terms, trying to make it seem like she had always been a part of the community, and it could seem like a bit of a mockery orotherwise just fake. So be authentic and aware in how you communicate.