There's something special about Nimrod that really resonates with British people. I always found use of it in films to be a rather cheap shot, but I suppose in Dunkirk it is just on the right side of tasteful.
I guess it hits extra hard in Dunkirk because of the ominous soundtrack that you're exposed through all the rest of the movie. Then with the plane safely landing in the end, the arrival of the trains full of soldiers, Churchill's speech and Nimrod placed over it, all that tension gets resolved at once in a beautiful way.
Not just British people. This American heard it (along with the rest of the variations) performed on my first date with my wife. It will always be a favorite.
[0] https://en.wikipedia.org/wiki/Benjamin_Wallfisch#As_composer...