The New York Consulate has a phone tree which says that you and your
fiance have to supply a statement saying that you both intend to marry within 6 months, and that you intend to live together as spouses in the United Kingdom.
I will be applying in person in New York, by the way.
Also, the first time I went over, I was questioned at the airport,
the second time I went right through without any questions at all
I think the first time was because I had never met my boyfriend in
person (internet romance) so he could have been some crazy internet person.
I have also never stayed for more than 2 weeks, which is perfectly normal for someone with a corporate job. My next visits will be
Thanksgiving and Christmas, when Americans normally travel.
Nobody has ever asked me anything about my job though, which isn't relevant because I'm not going for work-related reasons, but to see my boyfriend. I've always been straightforward about seeing him, giving his correct name, address and phone number on the landing form. I've also always returned to the US exactly on the day I told immigration I would return.