"You do not have to say anything. But it may harm your defence if you do not mention when questioned something which you later rely on in court. Anything you do say may be given in evidence
In the Royal Military Police 40 years ago we had a joke caution which was: "You are not obliged to say anything, but whatever you say will be taken down IN PENCIL, rubbed out, f**ked about and used to put you in jail".
It was all in good clean fun, of course:-)