What does Gary King become in the end? Is he happy now that he has found a purpose in life? Or is he simply content to repeat the Golden Mile crawl once again, this time with blanks? Why choose the kid blanks to team up with?
And The World's End is the most blatant example of it. At the end of the movie it is clear that Gary is a new man:
his idealized adolescence's visions have been beheaded (literally)
the place of his past glory has been destroyed (literally)
Gary King (of humans) has found a new sense of self worth by defeating the network
he doesn't rely on childish notions of manhood and self affirmation (alcohol is not needed to be a man, as he orders water)
So it is safe to say that the days obsessed with self indulgent nostalgia are over.
Why choose the kid blanks to team up with?
I am not sure that "why" is the right question to ask.
The ending makes sense/is satisfying because :
it ties nicely with the end of Shaun of the Dead where Shaun chose to hang with the villains of the movie (his zombified roomate)
it ties nicely with the whole premise of the movie (the Golden Mile) but now with a twist (well, two : the blanks and the water) to show the character growth.
it opens on a movie genre that the Cornetto trilogy hasn't touched : western (after zombie, cops and aliens movies). Edit Or, as Nobby points out, Mad Max post-apocalyptic film.
Gary graduated from obnoxious teenager to a man of honor. Instead of using his friends as mere means to have a second go at the Golden Mile, he now takes charge and fight for his mates (the Blanks that got bullied by the bar patrons).
Another perspective is from A. D. Jameson, especially his point 17:
Gary King doesn’t want to play by the modern’s world’s rules. But at the same time, he isn’t just playing by his own. He abides by a code: the code of companionship, of loyalty. His morality is arguably a medieval concept: the group identity of the band. Without his fellows, he’s lost—the archetypal melancholy wanderer of Anglo-Saxon poetry. But once everyone’s been reassembled—”Even Andy?” “Of course Andy!”—then King has a purpose, as does everyone else.
Gary needs a 'band' to have purpose, so he assembles one using the blanks.