BRU is a good alternate airport for DUS. Also you can try the overland route from LHR or CDG - Eurostar service from London to Brussels in 3h30 (Paris to Brussels in 1h30), then ICE from Brussels to Dusseldorf in a further 3h00 (you'll probably have to change in Cologne). Also bahn.de is currently offering a "Eurostar-London-Ticket" for travel between Cologne and London (not sure if it's valid the other way around).