Hiya
Well I don't know specifics about your type of situation becuase I've never been in one myself. However, reading up on the board and from hearing stuff from my family and friends I know this area of Customs & Immigration is nerve racking
I honestly feel USA customs officials are amongst the most sour faced, uninteresting, boring, monotoned individuals you'll see upon first arriving in America. To me, they are the ones who you REALLY have to be scared of! I'm sure UK customs fall into the same sort of catagory.
My suggestion to you, following from experiences I've had with USA customs is to be as confident, clear, positive and forthcoming to their questions as you possibly can. I know you'll be nervous inside, but try claming yourself down, breathing deeply and setting up your frame of mind accordingly 'in case' they ask you stuff. I remember visiting NYC again but for the first time since sept 11th. There was allsorts of scare stories about the already infamous US customs people giving more people an even harder time than usual. I was 'concerned' at the most ( remember I have brown skin and possibly easily mistaken for someone coming from the middle east!!!) I remember approaching the official and getting the usual smatterings of questions, and then the 'look' up and down, working me out, looking at my carry on luggage - he let me go with a monotone 'hava naace dayy' and have never ever since had any further problems. I'm sure them hearing a loudmouth semi cockney accent showed I was from London and not Lebanon or someplace! I walked through smiled and had a couple shots of JD whilst I waited for my cousins! - so I'm sure once you get through you'll feel like a weight's been lifted!
I can't advise what you should say and I'm sure other posters who know or have been in your situation can advise much better in terms of you having a boyfriend here and the implications of what you say in the long run for it - just keep your cool when you're speaking to them
Good luck!
Dennis! West London & Slough UK!! (oh and Heathrow is probably my 2nd home i'm there so dam often!)