You do not have to say anything. But it may harm your defence if you do not mention, when questioned, something which you rely on in court. Anything you do say can be given in evidence.

Above left, Police Sergeant Alfred Nash, circa 1900. Above right, obviously, not a police officer! Police Sergeant Alfred Nash would not recognise today’s police force.
The British Gazette does not condone the actions of Mr Matthew Doyle (the man who asked a Muslim woman at random to explain the Brussels terrorist attacks), but does regard the Metropolitan Police as having subjected him to politically aggravated harassment. Let us not forget, Mr Doyle spent three days in police custody. That is equivalent to a sentence of 6 days imprisonment. The Daily Telegraph is as good a source as any for more details.
GOTO: http://www.telegraph.co.uk/news/uknews/law-and-order/12204130/I-confronted-a-Muslim-tweet-suspect-no-longer-charge-with-race-hate-offence.html
What this case does reveal however is just how precarious the situation has become in the UK and England in particular. The authorities are (correctly) deeply worried at the stability of the multi-cultural, multi-ethnic, multi-faith, multi-lingual society that has developed. This combined with a massive housing shortage, strains on the infrastructure, unstoppable and unlimited immigration can variously be described as “a powder keg” or “a tinder box”.
Expect the police to become even more overzealous. Thus examples such as we reported on 25th October 2011 (http://www.british-gazette.co.uk/2011/10/25/politically-aggravated-harassment-mrs-jena-mason/) are likely to occur more often.
In particular, as Muslim numbers and the associated extremism increases, the authorities are likely to become ever more zealous in persecuting members of the ethnic Anglo-Saxon, Christian “community” to demonstrate “even-handedness” – which of course won’t be! Even handed we mean!
You have been warned!