Jack ate the beans, the beans were coffee, and it was all just a caffeine trip.
Coffee in England preceded this story by about 150 years. Had to do it though. Maybe Jack was the one who hadn't known about it being coffee yet, not England. So worth losing a cow. The Giant hates Englishmen because he is Celtic
Giants feature prominently in Scottish, Irish and Welsh folklore. And given that this story takes place long after the Saxons invaded the UK and oppressed the native Celts, this could explain the Giant's Anglophobia.