What she said, especially about the don't give any more info than asked for - just be honest and concise. The same goes for the information you carry with you...in all honesty, you may just be asked to show your return ticket. But, it's always good to carry the extra information with you in case you're asked for it.
If you are fairly sure you're going to go the fiancee visa route after your return from your visit in the UK, it's also a good idea to keep reading here (especially those links that geeta gave you) so you have good knowledge about what the rules are. When you say you're visiting your fiancee, you will also be able to say, if asked, that you KNOW you must return to the US to get the proper visa to settle. Again, you may be asked nothing at all, but it's good to be prepared.
MTA: You might also want to have it fairly fresh in your mind any sight-seeing you plan to do. Your visit is such a short one that you may not be questioned about your intent, but if you're visiting, it is to be assumed that you might go see some of the tourist sites!
Have a great visit!