so the two of you will be flying from the US to the UK together. he's on the return portion of his ticket and you're on a one-way ticket. you're only coming over here to see your boyfriend for a few weeks. right?
my two cents- i wouldn't lie. i'd be prepared for questions but wouldn't lie. do you every intention of returning to the us, right? you have a job in the uk, right? you have a lease/mortgage/etc in the us, right? i'd bring a letter from your employer saying you're employed, are going on holiday for xx days, and will be returning to work on xx-xx-06. i'd bring a copy of my lease/mortgage to show that you have a home to go back to. i'd bring copies of bank statemnets showing that you can support yourself by yourself for you entire stay in the UK. still- you'll be hassled. but lying to an immigration officer is never a good think (my opinion).