PM: I've noticed that this is a subject that causes you genuine and frequent annoyance, I'm not sure there's a great deal anyone can do about that of course and I've no desire to get into any form of argument.
What I will say is that I fly through EU and UK airports around 4 times a week and don't really find the level of surly attitudes that you do. I find it all pretty painless. It could possibly be how I approach the screening point, overall I find the people in security are pleasant when I'm pleasant. I'm not saying you're not, but I'm finding it difficult to understand why our experiences differ so much.
I do agree that there can be some small variance in interpretation, but airports (like airlines) are separate companies and so the situation is no different to your airline having an SOP for a given situation which differs slightly to another UK airline's SOP, even though they're both based on the same EASA reg. (Fuelling with passengers comes to mind - No airlines agree on that). I can't get annoyed about it because it's nothing that impacts on me. Armed police are exempt from screening when responding to an incident, (EU and UK regulation) but at other times *may* be patted down to ensure that there's no abuse of carriage of other prohibited items. (DfT guidance to airports, not a regulation). Either way, I'm not sure why you'd let that worry you.
At my last (UK) airline part of the job was liaising between carrier and airport to investigate instances where staff / crew had ended up in fractious situations, and I have to say that as the carrier representative it was frustrating and often embarrassing. The CCTV and audio evidence of incidents showed starkly that there are a minority of crew that will go into staff search actively looking for a fight. In 5 years there was only one altercation caused at the outset by an aggressive and rude security guard, sadly all the remainder (probably 15 cases) were caused by rude and aggressive crew, and naturally some other human beings will respond to that in kind.
I note that one of your major frustrations is not knowing the rules (I suspect it's the more detailed parts of regulation are what you're considering, since the basic rules are straightforward and understood by most pax). I just wonder if you'd benefit from approaching your airline's head of security with a list of stuff that's always bugged you? You might find that a chat explains things that you maybe currently have a half understanding of, and it might satisfy some of your questions that you pose on here, and hopefully get rid of some of those frustrations. As UK flight crew you're in a privileged position of having a specialist in your company who can support and help you, why not take advantage of that?