Surely the key phrase in that regulation is "a reasonable excuse". I would expect the police and courts to refer to the Government guidance when deciding whether or not a particular exercise outing is "reasonable" in terms of the regulation. So more than one outing per day, or a form of exercise that did not minimise the time outside the home, would be considered unreasonable unless there were exceptional circumstances.
Now I'm not a legal expert and the legislation I have the most experience with is social security but that's not how it reads as far as I'm able to interpret. Regulation six is comprised of four paragraphs and for purposes only paragraphs one and two are actually relevant.
Paragraph one tells us that: "During the emergency period, no person may leave the place where they are living without reasonable excuse." so far so simple. A plain English reading of that unarguably means that people are no longer able to leave home on a whim they have to have a "reasonable excuse".
Paragraph two is then the next most important for it tells us: "For the purposes of paragraph (1), a reasonable excuse includes the need..." and then lists a large number of different things. Note the wording "a reasonable excuse includes the need" that means that paragraph two is providing some definitions of a "reasonable excuse".
If we then consider the list of reasonable excuses the one relevant for our purposes is (b) which states: "to take exercise either alone or with other members of their household". Which means that to take exercise is a reasonable excuse which means that you are allowed to leave home to do so.
Now if (b) was phrased more like "to take limited exercise" or "to take the minimum necessary exercise" then I would agree with you that the plain English reading clearly infers limits and therefore the Government's guidance would seem a reasonable yardstick. But it isn't. "To take exercise" is simply defined as being a reasonable excuse no suggestion of any other consideration is implied. Taking exercise is a reasonable excuse for the purposes of paragraph one.
Like I said previously I would encourage everyone to stick to the Government's guidance on this as it's clearly the best thing we can all do for the sake of everyone.
But the law, in my view, as written, does not give that guidance legal weight. You cannot, in my view, be prosecuted under that law if you exceed the limits set by the guidance. Because the law does not match them. It is indeed silent on what the limits should be. I would argue had the Parliamentary intent been to proscribe the limits of exercise (or other factors) they would have chosen different wording to the quite explicit wording chosen.