Rachel, you're right.
You have no idea what is to come from your study abroad experience. You might speak the same language, but being abroad can sometimes feel like you're on another planet. Please soak up the last bit of American comfort but do know that the experience you're about to embark on is one that will have you telling stories for hours and one that will allow you to learn so much about yourself, other cultures, and your own culture. I have now studied abroad in London (Summer in London with IU Journalism) and Barcelona. I had a teacher tell me that to explain American culture to an American is like trying to explain water to a fish. They are so heavily immersed in their culture that they don't recognize that all actions and thoughts are contingent on the individuals' culture (whether that be British, Spanish, or American).
During your time abroad you'll start to replace some of your American reality TV for theirs (try the Geordie Shore or Made in Chelsea). As for fast food? You'll find Pret and EAT. You'll make new friends to text and call from your London phone. Radio? Brits listen to a lot of the same stuff Americans do--sometimes a few months behind but it's never an issue. Lastly, the car? When you get back to American you'll be look both ways to cross a street because you can't remember which way the cars are going to be coming from.
All in all, I'm sure your experience in London will be incredible just as mine was last summer. Keep an open mind and know that Bloomington's pizza joints and radio stations will be awaiting your return. Believe me, I'm excited to get back to some Scholar's Inn Bakehouse turkey sandwiches, Village Deli pancakes, and that IU spirit we all know and love.
Have a blast!