I don't think anyone has said it's against the law, just not adhering to guidelines (apologies if I am wrong and people actually have)
Unfortunately yeah they have, not sure about this thread, but I've seen people berated on Facebook and told they're breaking the law. There is nothing in the new law mentioning driving, or saying we can only exercise once a day. There is a huge difference between the 'guidance' and the Legislation. You can't get prosecuted or fined for breaking someone on Facebook's interpretation of the rules.
Having said all of that, yes of course I can see why the guidance is there. Its to discourage everyone from doing the same thing and converging on certain areas, making it unsafe for everyone.
As an example, we live near a local beauty spot which is always busy with cars parked so that people can go walking. Unfortunately, it continued to be full of cars after the lockdown. This made it unsafe for everyone, and although there's no actual law stating that people can't drive from the nearby town to get there, we were really relieved when the police started discouraging people from doing so.
But at the same time, I'm one of the people who drives 5 mins to a secluded area to walk. I do this with a full risk appraisal, I wouldn't park there if there was another car there, I'd move on and go somewhere else (within the parish, not beyond). I did this today, and drove past lots of walkers (including tourists, believe me we know who is local and who isn't) on the usual paths, so that confirmed to me that my drive up the hill was undoubtedly posing less risk to myself and to those people than if I had attempted to walk with them.