Wirelessly posted (BlackBerry8530/5.0.0.601 Profile/MIDP-2.1 Configuration/CLDC-1.1 VendorID/417)
It sounds like the OP only has enough miles for a one-way award, and is finding that the cost of a one-way paid ticket (ORD-LHR or LHR-ORD) costs considerably more than 1/2 the price of a roundtrip paid ticket on that particular route.