a dozen

I know that it is an equivalent of 'twelve' but sometimes it might mean just 'several' - I think. What if it was used in this BBC news bulletin like that:

"A dozen Yorkshire pits were targets for the worst concentrated violence of this dispute so far ... it was hit-and-run mob distruction ... Outside Hockleton colliery a crowd overturned two cars and set them on fire."

I would opt for 'twelve' but I'm not sure if it is possible at all to decide what was meant here.

It can be vague - "about a dozen" - though I wouldn't use 'several' for that many. 'Several' suggests up to about five, perhaps six; if 'a dozen' is being used vaguely (and I agree there is no way of telling in that sentence) it could be from ten to fifteen. But of course vague words are vague.

I would take as a rule, at least in UK English that a number is usually precise, and would otherwise be modified, eg 'about a dozen..', 'a dozen or so...' around a dozen...'. In the example given (as it is the BBC and so you are suggesting a reasonable level of correctness) I would expect there to be 12 pits, but take into consideration that it may be sloppily written and there may be more or less. As entangledbank says 'several' would suggest far fewer, but more than two. Also the lower the number the more likely it is to be precise.

I'd also presume it meant 'exactly 12' here too.

Over here in AE territory, I'd also assume that "a dozen" in a news report meant twelve, not some other nearby number. If eleven or thirteen pits were involved, I'd expect "about a dozen" or, better, "nearly a dozen" or "more than a dozen".