And your discussion of the legislation has been entirely accurate I think. However setting aside problems like a prosecutor having to reasonably think a claim above 50% likely to succeed (which is their threshold for taking a case forward), and their thinking it in the public interest to prosecute under the criminal law, neither of which I think they would believe (and which they'd be the subject of a successful complaint for being in breach of their rules over),I agree Jonathan, I'm aware of the existence of that treaty article. I've mainly been concentrating on the all too typical situation of a police prosecution going no further than a magistrate court initially, where no doubt the UK law could result in a conviction at that point.
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I don't think a prosecution could succeed, and the prosecutor's awareness of this would prevent them taking it forward.
This is because rules on carriages are covered by the treaties, the classifications are economic in their impact on intra-EU trade in the items concerned, and so subject to assessment for compliance with EU caselaw - assessment which the magistrate's clerk would be obliged to undertake, and advise the magistrate on. Or it might instead go straight to a crown court, which would be obliged to carry out the EU proportionality analysis, and in turn (just as with the magistrates' clerk) be obligated to find that the UK regulations could not be justified under the least restrictive option rule. The UK regulations would be disapplied there and then, and the EU caselaw position applied.
This would I think deal conclusively with the preliminary grounds of whether there is any legal basis for the prosecution, the conclusion being that there can not be.