The legal question is whether Cummings breached regulation 6 of the The Health Protection (Coronavirus, Restrictions) (England) Regulations 2020 as they stood at the time.
None of the listed reasonable excuses for leaving the place where he was living (his home in London) appear to apply. 
This point (presumably made by Cummings) is legally irrelevant (and wrong anyway - the guidance was, essentially, to stay at home).
This defence by Cummings (we all know what “a source close to” means: it’s just irritating) looks like an attempt to argue that he had a reasonable excuse other than those listed.
An obvious question that goes to this “reasonable excuse” defence: were there no reasonable alternative child care arrangements rather closer to home?
If the excuse is not “reasonable” he contravened regulation 6 and committed an offence: regulation 9(1)(b).
One obvious question for prosecution counsel would be: “if this was reasonable, and given your prominent role, why didn’t you open up about it before being forced to?”
