The US version ends with Sarah escaping the cave, running into the SUV and driving off. She then pulls over and Juno is next to her, obviously some sort of ghost that is haunting her now, like her daughter was earlier in the movie.
The UK version doesn't end there, though. Juno is screaming in the car and then Sarah wakes up and is still in the cave. She looks up and sees her daughter holding the birthday cake, which repeats the imagery that we are familiar with throughout the movie. Then the camera starts pulling back and we see that Sarah is just insane, the cake is really just the torch and there is no escape from the cave. Sarah's descent into madness is complete. In fact, it's also reasonable to make a logical leap that the monsters in the cave were not real at all and Sarah murdered all her friends, or that Sarah and Juno are the same person, but I think those theories produce more plot holes than they solve. I like the simple elegance that getting trapped in the dark, watching all her friends get killed by monsters after having her family die a year earlier just put Sarah over the edge. She no longer could mentally handle the situation and is doomed. Quite the dark prospect for the end of the movie but it makes it feel like a much more complete work, as well as making the title that much more relevant.
The UK version brings closure to the symbolism that the movie was presenting throughout the whole movie. After I was leaving the theater after the movie, I was puzzling about the ending quite a bit because so much seemed unanswered, as well as things being too convenient during her escape. Finding a way out after falling down another hole? Being able to find the cars without much difficulty even though coming out of the cave at a seemingly random location in the forest? That small cut makes such a big difference in the interpretation of the film. Everyone looks for different things in movies like these and just like any story with multiple endings, the "best ending" is never an easy thing to decide. But personally, I'm generally against altering the director's vision. There is too many examples of studios editing film and ruining it in the process.