I've got some posts on ignore so I may cover ground which has already been stated, but I wholeheartedly agree with noirem, you're not doing anything wrong and there's no need to lie, in fact lying can complicate things at later stages.
You have a return ticket, that's plus one in your favour, you have a reasonable reason to be in the country, you're not planning to over stay or work. It should be a fairly simple entry, if you lie you'll just look shifty and nervous, they'll quiz you, discover the truth and then wonder why you attempted to decieve them.
Merely state the truth - You've been offered a job at a UK company, before signing the contract you're coming to do touristy things and take a brief look a the housing market. Having an idea of what touristy things you want to do helps, like the London eye, walk around the area where your potential office would be, buckingham palace, a day trip to Brighton etc.
You understand that in order to accept the offer fully you'll need to return to the US and apply for your visa before re-entering. Have the offer letter with you but don't volunteer it unless they ask. If you're really worried about it, have some ties to the US proof, like a letter from your current employer stating you're on six days leave or your mortgage statement or something.