Ash I have been to the UK 6 times, all of those to visit my boyfriend now fiancee and I was NEVER detained or questioned past the usual.
Here is my advice to you..... When asked what is the purpose of your Visit, say Holiday (after all you are indeed on a holiday, right?). When asked where you will be staying, say with a Friend(s), in whatever city(after all you boyfriend is your friend, right?) When asked how long you will be saying, say 14days or 2 weeks.. which is a normal holiday for the UK so they shouldn't question it.
NEVER say that you are there to visit a boyfriend.. some may disagree but it will only lead to more questions and possible detainment if not being sent back.... but having said that you don't need to lie, if questioned further about your friend you are staying with, tell them honestly who he is, if questioned further tell them how/where you met, but there really is no need to tell them what kind of "friendship" you have or how serious your relationship is. (IMO)
Too many details or too much info offered(don't offer any info until asked) makes Passport control suspicious. Kept it short and sweet... if they ask you how you are when you first walk up to the desk, say you are tired.. after all you just came off of a Loooong flight and if/when you trip over your words (because you are nervous) you can blame it on being tired (because you are:D).
Good Luck and remember to have fun while in the UK
