I would phone the AA RTW desk - right now - and ask what they can do - (800) 247-3247, and see if they can revise the itin and get the rate desk to do things in a semi-hurry, then send the revised PNR to whatever ticket counter you want. It would probably help if you want to use AA metal transatlantic, but probably not mandatory as long as you use AA to get to the international gateway. LAS-DFW-LGW is probably the fastest; but not much quicker than other Mon. afternoon departures from other cities via DFW - SFO, LAX, ORD, SEA, YVR, JFK... Depends on how you want to spend the morning and early afternoon Monday - sitting in an airport or booking miles or SCs.