a friend visited the UK (can't remember where exactly) and he brought a case of Stella with him to a party/gathering, and everyone thought he was trying to get in a fight that night. any truth to Stella being associated with brawling?
Stella as someone else has said is known as wifebeater. Despite the brand trying to portray itself as a classy beverage, The majority of people see it the beer of choice for chavs, football hooligans, neo-nazi's, basically anyone whose poor, white english, and prone to violence at the drop of a hat.
Bring sweatshirts and jeans. if your friends moms son, came in the summer then they would have laughed at him, but trust me, its been snowing today, dress like you would to go to Oregon or Washington State as that has a similar climate to the UK.
The advice about pretending to be into football is a good one, but another way to get out of it is to look up your nearest MLS (the us soccer league) side and say you follow them.
Be wary in town centres around midnight-3am. Drunken pointless violence in these hours is a British speciality. I'm not sure if its just us as a nation who are like that.
Check out the local rock club nights. basically like a nightclub in the US except everyones dancing to punk, emo, metal and a bit of dubstep rather than mac miller. according to google your best haunt of a friday evening is this place http://theduchessyork.co.uk/whats-on/detail/720
You can also see the Bronx and Sonic Boom Six there this week and while she sleeps if you are still here in May.
finally go to Jorvik. seriously, you can't visit York without going to http://jorvik-viking-centre.co.uk/