I rented The World's End on Blu ray, having already seen it in the theater. One reason for seeing it again was to get a better look at the opening which was shot in super 16, cropped to 2.35.
It looks quite good. I'm sure the format was chosen because the opening is a story told in flashback.
The thing is, they used flash frames and uneven exposure to further contrast the footage from the rest of the anamorphic 35mm film.
What it tells me is that the super 16 is actually too good looking on it's own, so it required further embellishment to offset the look.
I suspect if the opening were left on it's own it would actually have cut well enough that it wouldn't have provided much contrast overall.