BA is at 0645 from Dusseldorf at present, which turns round at LCY to an 0735 return departure. Because of the hour's clock difference with continental Europe, and many flights being around an hour long, it is more effective for LCY aircraft to overnight in Europe to give early departures in both directions. Starting in the UK (as in the Cambridge example above) means the return flight can't leave until after 0900, which just is not competitive in this market, with many high-yield passengers doing day return trips.
Disappointing to hear of poor loads to Dusseldorf. I went some (maybe 10) years ago when VLM did the route, but they used Moenchengladbach instead of the main Dusseldorf airport, which was a tedious taxi journey from the city. Loads were always in single figures, I had better hopes for DUS itself.