The Boeing 777, carrying 272 passengers and 13 crew, turned back due to "fumes" reported in the aircraft's food preparation area.
The flight, which had reached as far as Milton Keynes, landed safely back at Heathrow and was met by three fire engines.
Following the incident, the flight was ultimately cancelled, with the aircraft remaining at Heathrow for an expected departure on Thursday 31 July.
This incident follows another recent United Airlines diversion on 25 July, where a flight from Washington Dulles to Munich returned due to a mechanical issue.