Can anyone post where the law says that you can't drive for exercise
The closest is part 14 here
And even that doesn't specifically ban driving for exercise. It says 'do not travel unnecessarily' and stay local. If someone lived in a very busy place, or had a dangerous road to get to an open space, or the example on another thread where the poster wanted to drive her disabled DH less than a mile so he could walk around on a flat area not the steep hill on which they lived, that he couldn't manage it would still be reasonable to drive a short distance to get somewhere more appropriate to exercise in most people's opinion.
The important thing when deciding what to do based on 'advice' published on the internet is to consider the reliability of what is written.
Regulations and words published on government websites are checked for accuracy. Not quite as reliable, but still quite reasonable, would be something like an official advice website, like citizens advice, going down the scale, you'd have quality media like the BBC, The Guardian or The Times, where it would either be clear that it is their opinion and you should check an official source, or they directly quote the government to illustrate the information they are trying to get across.
Then you have the tabloids who write all sorts of rubbish and use lots of words like could, may etc, to indicate a caveat that the truth could be nothing of the sort and below all that you have public forums where anonymous posters can write exactly what they like whether it is true or not.
I could write 'My name is Michael Gove and you can take one hour of exercise per day no more, no less' and no-one would have any clue as to whether that was true or not and would be very unwise to take that statement as fact.