What disadvantages would it have brought to airlines or anyone else if the LHR curfew were extended to 07:00 while the airport is so little used? Is it just they don't want residents of West London to get used to sleeping with the windows open?
It would risk the residents or other groups then trying to make 7am being the permanent curfew lifting time. "If you managed in April, then you can do it in October" etc...