Some things are obviously pranks, but I can buy their side of the story. I'm not sure what the specifics of plate registration are in Argentina, and the explanation was not terribly illustrative, but suffice it to say that this appears to be their reputation catching up to them, and perhaps not deliberate provocation.
In any case, it is hilarious. I love their style, and I'm glad that they're willing to ruffle feathers in pursuit of it. Most people never have the opportunity to cause international incidents with their caricatures, and I'm glad that Top Gear takes their responsibility seriously. A little mockery on the international stage helps defray larger tensions, and no one can be held accountable for what Top Gear does but the show itself.