Return ticket necessary?

I am planning on coming to BA for about 6 months and on taking a trip to Uruguay at 90 days to renew my passport. Am I required to show a return flight ticket when I first arrive in BA? In which case should I buy a refundable ticket and then cancel it when I get there?
Munch,I came down on a 1 way ticket without any return ticket. It is possible. I would have a story prepared about how you intend to travel South America and are unsure of where you will be departing home from.

Now when I travel back to the US, I buy round-trip flights originating in BA. The only problem with this is that I can't use Orbitz or Expedia to look at prices. They only work for trips originating in the US. Good luck. Any other questions, feel free to contact me directly.