Here is the guidance:
https://www.gov.uk/government/publications/covid-19-stay-at-home-guidance/stay-at-home-guidance-for-households-with-possible-coronavirus-covid-19-infection
The key paragraph is this one: if you live with others and you are the first in the household to have symptoms of coronavirus (COVID-19), then you *must* stay at home for at least 7 days, but all other household members who remain well *must* stay at home and not leave the house for 14 days. The 14-day period starts from the day when the first person in the house became ill.
The two *must*s in that paragraph are important. People have been fined for not following that instruction. In fact, they've been fined for far less.
Cummings did not follow that instruction. This is beyond question. Nobody has claimed that he and his family did not travel from London to Durham. What has actually been claimed, by the Prime Minister no less, was that Cummings had "no alternative" but to make that trip. That's OK then, isn't it? "Must" did not mean "must", clearly.
I mean,
an alternative couldn't have been that one of the perfectly well members of his family from Durham came and collected his child should he reach the point when he could no longer cope? Which, as it turned out, he did not reach.
This is classic doublespeak. Must means must, except when you have "no alternative" but to do something that suits you better. Boris has just given the whole country permission to do what the heck it likes to deal with coronavirus. Having conceded the English language to his special adviser in order to appease him, for reasons unknown (I'd love to know!), Boris's leadership is more like Chamberlain's than Churchill's.