Helicopters for the US contingent, including Marine 1 if required, are likely to be delivered via Fairford, where the food is more American. The Ospreys don't need carrying, although I'm not sure I would like to do trans-Atlantic in one.
When President Obama attended the NATO summit at the Celtic Manor in Newport, Air Force One flew into Fairford and a whole array of smaller aircraft (including the Ospreys and Marine One) flew out about an hour later, presumably heading for Cardiff.