First of all, I should say that I'm not British, so I can't help you specifically with you choice of A-levels. I can however try to indicate the sort of skills that are likely to go down well with an admissions committee. You are also welcome to PM me.
I think the most important thing is to tailor your choices to the principle underlying the ASNC course. It is interdisciplinary, in that it doesn't focus on a particular discipline, but rather on culture, and then uses a variety of disciplines to study culture. Choose subjects that are interrelated, (and which will be relevant when studying early medieval culture) e.g. history, languages, literature, archaeology, and philosophy.
Since the history and literature covered in ASNC are rarely studied in depth at school, you need to show positively that you are knowledgeable about it, and that you have a real interest in it. I also think that ASNC is one of the courses where extra-curricular activities are most important. I would suggest reading at least some Icelandic sagas, Bedes Historia Ecclesiastica, Beowulf, and the Mabinogion. Think about their value as historical sources, and their value as literary products of the relevant cultures.
Good Luck!