Düsseldorf and its surrounding area has always had strong air links to the UK, there are more uk destinations served from Düsseldorf than any other German airport. Newquay has links to Düsseldorf, for a time Inverness had a Düsseldorf link I believe. The UKs armed forces presence has always been in the North Rhine-Westphalia religion, which the capital of that state is Düsseldorf which has helped with those links. Frankfurt was up until 2005 the main Air Force base for the US Air Force in Germany, links weren't as strong to that region, and while it now is Lufthansa largest hub, we were just down the pecking order. Even now, I counted 7 UK destinations from Frankfurt but 13 from Düsseldorf.