Airport hassels in London?

I will be traveling from the US to London and then on to Paris. I have only purchased a one way ticket, as I do not know the exact length of my stay, and will simply buy a ticket home when I am ready. I am planning on flying in to London Heathrow airport and then by train to Paris. I have been told by a few people that without a return ticket I may have problems convincing the authorities that I will not be staying in the country longer than I am legally allowed to do. What exactly do they look for? Am I required to produce proof of a return ticket or maybe a train ticket out of the country? Any help or advice appreciated. Thanks.

Taking a moment's reflection, jason mused: I will be traveling from the US to London and then on to Paris. I have only purchased a one way ticket, as I do not know the exact length of my stay, and will simply buy a ticket home when I am ready. I am planning on flying in to London Heathrow airport and then by train to Paris. I have been told by a few people that without a return ticket I may have problems convincing the authorities that I will not be staying in the country longer than I am legally allowed to do. What exactly do they look for? Am I required to produce proof of a return ticket or maybe a train ticket out of the country? Any help or advice appreciated. Thanks.

When I flew into Heathrow last year, the simply asked for my passport and how long I was staying. They asked my occupation, and how much money I had with me (to determine the likelihood of me seeking employment I suppose). The issue of my flight arrangements never came up.

Probably you won't have any trouble - no one I know has ever been turned away for not having a ticket for an onward journey. But if you've already decided you want to go to France, why not buy a train ticket in advance and have that to show in case the question comes up? You can check the Eurostar web site and see if it's possible to buy a ticket and have it sent to you in the US:
http://www.eurostar.com/

If they won't do that, you could probably just show a copy of your confirmation email. However, I don't really think you need to worry about it at all.

Have a great trip!

Other people have provided reasonable advice. Just one other thing, which may seem obvious: Don't knuckle under to internal pressure and volunteer that you have a one-way ticket unless asked directly (in which case you should answer honestly). If they ask how long you plan to stay, just tell them that; don't bring up the ticket.