You will be just fine! September is a great time to non-rev and like the others have said, there are tons of USA-London flights. If you ZED, you will not need a back up. Don't buy tickets when you can get them for free, well almost free.
I have not "paid" for a ticket in years and we even non-reved on our honeymoon. I have non-reved over 850,000 miles without getting stuck anywhere.