I think that Delta will be handled by Air France Services Limited as they will be checking all Delta and Air France flights to USA at the same desks. Delta and Air France will operate jointly between Heathrow and the US: JFK and Atlanta operated by Delta aircraft and crew and Los Angeles operated by Air France aircraft and crew.
As this is a strong partnership I'd be surprised if it did not include the ground handling too.
US Airways is member of Star Alliance, so I'd say United but they fly to a different terminal. So maybe Menzies, Aviance etc.