They might not be Cannes winners yet, but over at the Viacom Outdoor, Underground website there is a large collection of PDF's of ads competing in the underground writing competition.
Almost all entries are there, sadly mainly made available as downloadable PDF's which can seem a tad annoying at first, but they probably did that to let the art direction be.
See also Crack a smile, Harrod's tea story, national blood service
... and many more.