//I am assuming some one who finds themselves in the above position would now not be able to present the above evidence in their defence – as they were unaware of it at the time of the police interview.//
You seem to misunderstand the purpose of the “PACE” warning, Hymie. It does not warn the suspect that he cannot mention something in court which he did not mention in interview. It says that if he does his defence may (but not necessarily will) be harmed. The Rules of Evidence and Criminal Procedure Rules (which govern how a trial is conducted) did not change with the introduction of PACE.
The case of your friend’s wedding is a perfect example of why a court may be sceptical about what they are being told. They might well think that when questioned he had no explanation for why he may have been over the limit (other than the fact that he’d drunk too much). Between then and facing a court he may have had a chance to explain the situation to his brother. There are then two possibilities: (a) his brother comes clean and explains what he did or (b) between the two of them they concoct a story to say that the brother will say he spiked his drinks and will agree to perjure himself in court. The court would have to decide which version they believe.
Your other example (finding the diary) is similarly not affected by the PACE warning. Where you are likely to come unstuck (as Buenchico kindly pointed out) is under the Criminal Procedure rules and accompanying legislation. If you intend to defend a matter in court you must have your defence prepared by the time of your trial. Trials do not occur within days. Even at the best of times the most straightforward matter takes weeks to reach trial day. Similarly convictions cannot be left “open” indefinitely “just in case” any new evidence comes to light. But once again, this has nothing to do with the PACE warning.
PACE was introduced largely to ensure the police followed correct procedures when dealing wit suspects. The introduction to PACE says it is “An Act to make further provision in relation to the powers and duties of the police, persons in police detention, criminal evidence, police discipline and complaints against the police..” Before its introduction there were no fixed rules about interviews, recording them, etc. In fact, it was largely introduced because throughout the sixties, seventies and early eighties there were numerous high profile cases of suspects being “fitted up” through the falsification of evidence, concocted admissions and so on. Perhaps the most infamous of these led to the “George Davis is Innocent” campaign in the 1970s:
https://flashbak.com/george-davis-is-innocent-the-1970s-campaign-that-proved-the-police-and-state-were-not-ok-2408/
Most of its provisions were aimed at ensuring suspects were treated consistently and fairly. The now famous warning was seen as a requirement because suspects had the right to remain silent (or provide a “No Comment” interview). But judges and magistrates were (and still are) entitled to draw adverse inferences in certain circumstances from such a course. All PACE did was to ensure that the suspect was made aware of this. Nothing else changed and your fears that the warning itself places constraints on the suspect’s defence are unfounded.