How do you say hello? which leads to - who are you?

When I started working here I was called Paní učitelka, Mrs. Teacher. Young private students call me Jennifer by my request, and high school students call me Mrs. Jennifer by their school policy. Non-native English speaking uni students address me in writing as Dear Miss Jennifer or Dear Miss (for speech drop the Dear), American students almost always go straight to Jen, and my local work colleagues call me Jennifer unless we're friendly and then it's Jen. (Their emails will start off either Dear Jen(nifer) or Hi Jen(nifer) or Jen(nifer).)

I'm dealing more and more with British people, and I don't know if the ones I'm dealing with in particular are a bit more Americanized, or if the whole country has suddenly gone informal, but it's been pretty funny this past week watching the evolution/deterioration of their chosen form of address... Keep in mind these are all strangers, they work together at a uni, and I close all correspondence with my full first name.

Dear Mrs XXX, Dear Jennifer, Dear Jenny, Dear Jen, Hi Jenny, Hi Jen, Hey Jen, Hiya!, and then a complete lack of salutation all together.

I participate in it in a different arena... if I meet you through work it's Jennifer. If I meet someone I don't really want to know - bad energy, horrible reputation, off putting personality, my own foul mood or discomfort with the dynamics - I introduce myself as Jennifer, if I've just come from work and haven't mentally switched yet or am otherwise distracted, it's Jennifer - but if I've met someone and they're friendly and friends with my friends, it's Jen, right away. Usually the transition takes place with a level of familiarity, but I guess the introduction in my chosen name rather than my given name can also be considered an invitation to familiarity. Now don't go try to analyze how I introduced myself to you when we met. I'm too moody for you to get anywhere with that!