When talking to officials, whether private or public, remember the first two sentences from the Miranda Rights blurb parroted by cops in the US:
"You have the right to remain silent. Anything you say can and will be used against you in a court of law."
That's pretty good advice in many circumstances.
Here's a short compilation of someone putting that into practice:
And here's a law professor and a police detective elaborating on why this is a good approach:
In the UK, important case law on this point comes from Rice v Connolly [1966] 2 QB 414, wherein it was determined that, where there isn't a specific duty under legislation for citizens to assist police (which is most circumstances), they are not required to do so. Consequently, there's no need to even talk to them.
In that case, Chief Justice Marshall stated:
" It seems to me quite clear that though every citizen has a moral duty or, if you like, a social duty to assist the police, there is no legal duty to that effect, and indeed the whole basis of the common law is that right of the individual to refuse to answer questions put to him by persons in authority, and a refusal to accompany those in authority to any particular place, short, of course, of arrest."
However, an important distinction was made, that:
" In my judgment there is all the difference in the world between deliberately telling a false story, something which on no view a citizen has a right to do, and preserving silence or refusing to answer, something which he has every right to do."