Surely it's not just the distance alone that's involved anyway. The relevant Regulations (both in England and Wales) state that "During the emergency period, no person may leave the place where they are living without reasonable excuse". The Regulations also include a non-exhaustive list of what can constitute 'a reasonable excuse' but in circumstances which are not covered by those specific exemptions it will be up to the police initially, and then the courts if fixed penalties are challenged, to determine what does, or does not, constitute 'a reasonable excuse'.
Obtaining food from a
local takeaway outlet, for example, would seem to fall within the definition of 'a reasonable excuse' but it seems understandable to me that the Old Bill don't regard driving very long distances to get to a branch of McDonald's as providing 'a reasonable excuse' for leaving home.
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https://www.theanswerbank.co.uk/ChatterBank/Question1736739.html )
i.e. an offence is committed the moment someone leaves their home, rather than when they get to their destination, if they leave without a reasonable excuse.
Similarly, nipping down to your local corner shop for a pint of milk, or possibly driving 5 miles to the nearest supermarket for a 'big shop' that includes one, is almost certainly a reasonable excuse for leaving home but (unless you live in a really rural location, with no local shops or supermarkets) leaving home with the intention of driving 20 miles to collect some milk isn't. (i.e. if a person so doing is stopped only 100 yards from their house, and admits that they're setting out on such a journey, they've ALREADY committed an offence through leaving their home without reasonable excuse).