In reply to Anonymous:
Becasue garden centres are distributed throughout the country pretty well matching population distribution and local demand. So although you can drive a long way to go to one, very few people will. This means there is very little benefit to restricting it. The cost-benefit calculation is not in favour.
Nice walking areas are not distributed like that, and before the coronavirus people routinely travelled longer distances, and gathered in larger numbers at popular sites. This is undesirable from the point of view of creating new clusters in areas which were otherwise pretty OK. It's also not great in terms of the risk to the people who need to pick up litter, clean public toilets etc. after the masses have been and gone. So we want to restrict that a bit, the cost benefit calculation is in favour, even if there is some collateral damage.
We could have a "no honeypotting" rule. Or we could ask everyone to do a written risk assessment before heading out and trust them not to let their desire to get out cloud their judgement. But frankly I very much doubt that would work. So we get the 5-mile guideline. I'm not sure I agree with it, but I can see a scientific case for it.
Post edited at 11:21