This august I am planning to travel around Europe and want to cross the French-UK border. I am under 18 and won't be with my parents or legal guardian, so what kind of documents do I need to have with me? I'm a Lithuanian citizen.
Nothing special. I did this not too long ago in Europe personally. You just need your passport/travel document, and a valid explanation as to where you are traveling and why you are doing it alone if you are questioned about your age. The contact details of your legal guardians would also be wise to have on you in case the border guards want to call and verify, I personally had this happen when I went to Canada previously, they called my parents and the people I was staying with.