Getting an internship in the UK?

I want to try to get an internship in the UK for this summer. I previously went to school in Canterbury; Kent, and now attend the University of Illinois. I was going to fly out and try to meet with people, get portfolio feed back and what not sometime this spring. I need to know how much of a heads up should I give the people/ firms I want to talk to? Should I offer to take them out for coffee, or lunch? Which days of the week would be the best to talk to people on?