Go for the RT fare; done that a good few times.
Do your return flight a few months out - about the time you might visit the US. You might use it, or worst case, you dump it. I once use a return bit for a quick trip to the US - had booked a return >6 months after the UK flight. I actually managed to find a cheap one-way flight to come back to the UK, so it all worked out nicely.