Re: Health Secretary Matt Hancock, has been found to have acted unlawfully
Not exactly the same, but: There are findable explanations on the difference between "illegal" and "unlawful". As I read it, it seems "unlawful" is used when a thing is against the law but not specifically forbidden (e.g. in some countries owning a gun might be a specific, forbidden thing, and therefore illegal). Perhaps you might execute a variety of manoeuvres in your car that are not explicitly listed anywhere as "dangerous driving", but that, being dangerous, are unlawful (but not specifically illegal).
My *guess* therefore is that "acting unlawfully" means something like "breaking the law, but in a way that is not trivial to prove".
This being an IT site, there will no doubt be a lot of lawyers reading who can and will tell me I'm utterly wrong shortly. :-)