I await comments from those who earlier presented themselves as experts on the current flawed legal system, but I'd argue that passing a marshalled "ROAD CLOSED" barrier is "obviously dangerous" every time.
A good run down of careless vs dangerous is here:
https://www.thelawsuperstore.co.uk/.../what-counts-as-careless-or-dangerous-driving
Essentially it agrees (as do I) with what you say. If deliberately driving through a road closed sign and then deliberately driving at the crowd isn't dangerous driving, I'm not sure what is. The only thing I can guess at is that the Police/CPS don't feel they have enough evidence to go for a dangerous charge rather than a careless charge. I think sometimes the rationale is that it's better to succeed with a charge of careless driving than fail to get a successful prosecution for dangerous driving. There doesn't seem to be a hierarchical fallback that if dangerous driving isn't proven, it couldn't then be considered careless driving.