"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"

That appears to be the Warning.

I am not a UK attorney, but this is what I guess it could mean.

You are arrested for doing something, but have an alibi or explanation for your act, (far-fetched but) your identical twin brother actually did it. When yor arrested and a witness points you out, you do not say tell the police, that your twin brother did it. Now at trial say 6 month to a year later,you tell the court or use as a defence that your twin brother did it. The prosecutor can attack your credibility for not raising it earlier. Perhaps (I do not know UK Criminal Procedure) you might not even be able to raise that defence at trial having waived it at the time of first questioning, or the Judge may direct the jury to ignore the defencd or give it no weight.

Perhaps in the Kaylee Anthony case, a warning like that given to the mother when she was arrested, would have denied her some of her defence.

Rami

Older version:
"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."
Newer version:
"You do not have to say anything unless you wish to do so, but I must warn you that if you fail to mention any fact which you rely on in your defence in court, your failure to take this opportunity to mention it may be treated in court as supporting any relevant evidence against you. If you do wish to say anything, what you say may be given in evidence."

"Further the invoking of that right does not prohibit officers from asking further questions."

A second Wikipeida article:
England and Wales
Main article: Right to silence in England and Wales

The right to silence has a long history in England and Wales, first having been codified in the Judges' Rules in 1912. A defendant in a criminal trial has a choice whether or not to give evidence in the proceedings. Further, there is no general duty to assist the police with their inquiries.

At common law, and particularly following the passing of the Police and Criminal Evidence Act 1984 under Code C adverse inferences may be drawn in certain circumstances where the accused:

* fails to mention any fact which he later relies upon and which in the circumstances at the time the accused could reasonably be expected to mention;
* fails to give evidence at trial or answer any question;
* fails to account on arrest for objects, substances or marks on his person, clothing or footwear, in his possession, or in the place where he is arrested; or
* fails to account on arrest for his presence at a place.

There may be no conviction based wholly on silence.[6] Where inferences may be drawn from silence, the court must direct the jury as to the limits to the inferences which may properly be drawn from silence.[7]

This does not apply to investigations by the Serious Fraud Office, where there is no right to silence, nor in relation to terrorism.

Under Section 49[8] and Section 53[9] of the Regulation of Investigatory Powers Act 2000 (RIPA), it is an offence to fail to disclose when requested the key to encrypted data (with a penalty of two years in prison).