Miranda rights vs the caution
If you've been a good boy or girl, the only time you would come across the Miranda warning (or, as it is more frequently known, the Miranda Rights) or the Police Caution on the dozens of cop shows on TV. Both of these warnings are given as soon as possible after someone has been arrested. On telly, the accused will often reply with a poignant "Sod off, wanker" or similar. Come to think of it, the same happens in real life.
Anyway, on TV, these warnings are often rattled off at incredibly high speed. If you take a closer look, however, you'll see that the two apparently similar warnings are different. Very different. The Miranda rights in the US and the Police Caution in the UK are, in fact, the exact opposites of each other.
The Miranda Warning
A quick refresher, in case you aren't completely up to speed with your cop shows:
You have the right to remain silent. Anything you say can and will be used against you in a court of law. You have the right to speak to an attorney. If you cannot afford an attorney, one will be appointed to you.
To break it down:
"You have the right to remain silent" is pretty clear. Basically, it's cop-speak for "shut the hell up".
"Anything you say can and will be used against you in a court of law" outlines the consequences of not listening to the first bit: If you don't shut the hell up, we'll write down anything you say, and find a way of using it against you later.
"You have the right to speak to an attorney. If you cannot afford an attorney, one will be appointed to you." is also pretty clear. It means "Get professional help before you talk to us. If you don't have a lawyer, we'll find you one."
So, to translate the whole phrase:
Shut the hell up. No, really, shut the hell up, nothing good will come of talking to us. Get a lawyer before you say anything.
The Police Caution
The police caution, in all its grammatical clumsiness, is as follows:
You don't have to say anything, but it may harm your defence if, when questioned, you fail to mention something you later rely on in court. Anything you do say may be given in evidence.
Wow, that's a bit of a mouthful, but let's do as above and translate it:
"You don't have to say anything" is pretty clear. Keep your mouth shut if you want to. (But it's worth noting that this is not an explicit right as such)
"but it may harm your defence if, when questioned, you fail to mention something you later rely on in court" means something completely different than not having to say anything. What we are saying here is that "If you have anything to say in your defence, you should bring it up when you are being questioned, rather than in the courtroom".
The inference is that if you forget to explain something that's integral to the thing you are being accused of (like "I wasn't breaking into the house, I was housesitting for my neighbour and locked myself out. I had to break in so I could feed the cat"), that we will use your silence against you. It's easy to see how this could transpire in a courtroom: "Really? You were housesitting? Why didn't you tell the police officers about that when they arrested you?".
"Anything you do say may be given in evidence" basically explains what happens if you say anything.
So, again, translated;
You can keep your mouth shut if you want, but if you don't tell us everything relevant to the case, there's no use in crying about it in court. Oh, and we can repeat anything you say in court.
What's the big deal?
So, let's take a look at the translated phrases:
In the US, they tell you "Shut the hell up. No, really, shut the hell up, nothing good will come of talking to us. Get a lawyer before you say anything."
Meanwhile, on the other side of the pond, it's more like "You can keep your mouth shut if you want, but if you don't tell us everything relevant to the case, there's no use in crying about it in court. Oh, and we can repeat anything you say in court."
The quirk here, of course, is that in the UK, "When Questioned" will usually be at a police station, with a tape recorder running. At this point, you're offered legal assistance, and you are perfectly entitled to say nothing until you have a solicitor present. UK police guidelines are to not do any questioning between arrest and the formal interview at the police station, unless they are urgent questions relevant to the case. ("On which road did you hit the child, we have to send an ambulance" or "Was there anybody with you in the car that's now sinking to the bottom of the lake").
The main difference, then, is that in one country, they'll tell you to keep your trap shut, while in the other country, they'll tell you of the bad things that may happen to you if you do so.
It's funny, how two relatively similar societies can be so different on something this basic.