https://en.wikipedia.org/wiki/Right_to_silence_in_England_an... is a good summary. You can claim it later, although the jury are entitled to infer from your not mentioning it earlier that you've made it up since the initial questioning. It has to be related to factual matters which you're actually questioned about. You're entitled to shut up until you get your lawyer present for the police questioning, but at that point you have to have your story straight.

A longer discussion: http://www.publications.parliament.uk/pa/ld200304/ldjudgmt/j...