Normally you'll need around a B in History, but it may be more or less depending on how competitive the 6th form is and their standards. Remember that 6th forms are sometimes flexible on entry requirements.
Generally, when they ask for a B in English Language, but the 6th form may be willing to accept a B in Literature instead. It all depends on the 6th form.
I'm currently in Year 13 doing A2 History, and I love it, but then I've been interested in History for as long as I can remember and I'm going to be studying it at university next year. Do you enjoy History at GCSE? How do you feel about essay writing? What do you want to do in the future? What topics will your 6th form be covering? Try asking yourself these questions, because everything thinks differently.