@crowriver: according to Gogol Maps, it's 7,4 miles from Ashby-de-la-Zouch to Foremark Reservoir by road
Setting aside for the moment the fact that this discussion is purely theoretical since the parties concerned were subject to English law, not Scottish, I specifically said in my post that the reservoir is 5.2 miles from the border between North West Leicestershire and South Derbyshire. And I said that because the Scottish regulations (which, per my post above, are still in effect) restrict such trips to "within 5 miles of such local government area" not five miles from where the journey started.
I do wonder whether the widespread publicity given to this case will encourage all manner of "enthusiastic exercisers" to chance their arms on driving distances to go for a walk.
In Scotland, as things stand at the moment, it's legal to do so within the five mile rule, no arm chancing required. As for driving separately, which you also criticised: current guidance is to avoid travelling in the same car as someone who is not from your own household if possible.