Well, I only visit the UK for short periods of time, so my experiences are probably different than that of someone who is living in the UK. The first time I came back into the US in Detroit, the immigration officer just asked me about how much I had spent in the UK (money wise), and what I did for a living.
The second time, the I.O. at Philly airport asked me why I was in the UK, what my relationship was with the individual I was visiting (at that time, he was just a friend), did i ever see myself falling in love with him, how frequently he comes to see me (at that point, "he hasn't yet"), why he doesn't come to the US as much as I go to the UK ("It's easier for me to get vacation time, and I have an easier time affording the trips"), if either of us planned on moving to the others country ("Not at the current time..." was my answer at that time), why I had been taken into secondary inspection in the UK, what I did for a living, etc.... basically, so many questions, all because of a two week trip!
After he stamped my passport, I asked him if there was a reason that I was asked so many questions, when I saw so many people walking right through. His response? "It's my job to ask questions. You didn't do anything wrong, I just have to ask." He also stamped my passport with an entry stamp -- which wasn't done the first time. I asked him about that, too, and he told me that I should always get a stamp when I return home.
I kinda got the idea that he might've been new on the job, and trying to be very very diligent. He was fairly sweet for an immigration officer... he just asked more questions than I was even asked during secondary inspection in the UK!