Berkeley Hunt is rhythm slang for the word, 'cunt'. It is a way of saying what you're sometimes actually thinking without offending those around you.
For example, you're in the middle of a busy supermarket when a little kid runs into the back of your heels with a shopping trolley. Instead of actually saying cunt, you turn round and say, "aaahhhh, you little berkeley hunt!". This way no one is offended and you feel better.
Another example would be ask a woman to get her berkeley hunt out if you wanted a bit of 'business' off of her!