I'm glad that you have brought up the topic of the interchange flights operated by Delta and Northwest at LHR in the early 1970's. I knew that it was getting to a point where someone was going to bring it up. There was probably only two or three posters that was going to bring the subject up with yourself included and maybe myself. I was actually contemplating it myself but it's a fairly complicated story to tell but again just like your other posts you have explained the situation in great detail.
Although I'm totally aware of those interchange flights by Delta and Northwest at LHR during that period in time I personally never saw any of their aircraft as it was before my time. I wasn't quite into aviation then, that didn't occur until 1977.
I don't think that Braniff ever visited LHR as part of the interchange agreement as you rightly pointed out.
There are numerous photos online and in particular on Flickr showing the Delta B747's at LHR during that period plus a few of the Northwest B707's.
Not sure of the schedules regarding the Delta and Northwest aircraft but they were more than likely on the ground at LHR at the same time. I wonder if there is a photo somewhere showing the Delta B747, Northwest B707 with a National DC8/B747 alongside the Pan Am and TWA aircraft at LHR Terminal 3. I'd love to see that ☺️
I have to say that the Delta livery looked great on their B747-100's especially at LHR alongside all of the other airlines of that period.
I really think that this thread is better suited in the History and Nostalgia forum as we've gone well past the original topic of Pan Am flights at London Gatwick. But it's been a fascinating thread and I'm enjoying being part of it.