If the loads are light coming back to IAH, they'll simply amalgamate those return flights also, thus saving the marginal costs of flying a 777 to, and landing at, LHR and back. There might be some EU261 compensation to pay, but many of the US based passengers won't know about that, so the amount will be trivial. The only strange thing in the story is why they were flying the crew over as there should have been spare crew in LHR who could simply have a day off and wait for the next day's flight.