Fortunately, you definitely can write your UP Board class 12 exams in English medium even if yo are admitted to an Hindi medium school, the UP Board is very flexible and allows students to choose their prefer medium of instructions for the board exam regardless whatever medium of school they are in meaning that even if you are in a Hindi medium school you can give your answers in English. However, it is  important to notify your school about the language in which you will be writing your answers. And  you must  consult with your schools administration they will provide you more information.