Emigrating to US..One way or Return ticket?

I'm moving to the United States from the UK early next year and I've been looking at the flight prices.As I wont be coming back for a long time I would need a one way ticket which is about �640,but a return is about �200 cheaper.Would I be allowed to get the return ticket and cancel the return portion once I'm in America?.I just wondered what the written rule is taking into account I'm going there to live and didn't want to break any flight rules or whatever,any help I receive will be much appreciated..Thanks :-)