"The fact" isn't necessary here because it's redundant. If you hadn't know about something, what you hadn't known about is a fact. This sentence, in the most concise way I can think of, should read:
"I was totally shocked by the northern dialects and other British accents, because I hadn't known that these kinds of words existed before I went there."
I hope my answer helps. Please let me know if you still have questions =)