In my eyes there should be clear guidance, of say 10 miles from your home address to travel for exercise purposes.
If a limit is to be considered, I’d suggest it should be measured in expected travel time rather than distance.
Having given this some thought, the police are in a difficult situation. Extremely authoritarian law has been passed and they can either not enforce it, or try and enforce it. The more common - and wiser - approach is the former, but other forces try and “square the circle” by enforcing.
The problem is that the way the law has been written (to ban people from being outside of their house, except for excuses), the only way such enforcement can happen is via an authoritarian police state. It is a suspension of the default position of freedom, replaced with a default assumption of breaking the law subject to an excuse, which can only be determined by questioning. The U.K. police have no stop and account power and must suspect you of being outside your home without reasonable excuse. But failing to engage with police would likely be viewed as reasonable grounds for suspicion by many officers. The end result is the same.
Other places that have implemented superficially similar lockdowns have not done the same. The Netherlands for example never banned being outside your home or travel, just closures and household mixing limits. Given what a fundamental shift this is, the onus is on lockdown proponents to explain why the incremental benefit justifies such a fundamental stripping away of rights.
I expect the argument would be that banning being outside the home makes it easier to enforce against things like household mixing. But we do not routinely strip populations of fundamental freedoms just to make enforcement easier - even for much more serious crimes we could possibly prevent. Now we’ve set the bar for this unprecedented step at “preventing risk of passing on a disease with an overall mortality around 1%” (this is not to underplay it), why shouldn’t we remove other fundamental freedoms (like the right to privacy) to prevent murders, pedophiles, etc? While we’re at it, why not keep the “no person may leave home without reasonable excuse” law indefinitely, just expanding the list of reasonable excuses, so we can then question anyone loitering for any reason; we’d probably catch all sorts of otherwise hard to enforce criminal activity?
