Bluetrees123 - you're good value, I will say that for you.
As I've said, I'm overseas & private & things are quite different; we'd just decline your business. My school wouldn't be a good fit for you, because we have expectations you aren't willing to meet, so we'd explain that we couldn't continue to offer places to your dc, & off you'd trot to find another school that had different expectations & suited you better.
Our parents want, & pay handsomely for, a top flight education & high expectations - starting with 'everyone is on time & their lessons aren't disrupted by students straggling in late'.
I taught in the U.K. system for nearly 20 years before bailing for the sunlit uplands of a lovely tax free salary &, frankly, fabulous opportunities for my own dc.
U.K. state schools are bedevilled by all sorts of factors that make it hard or impossible for them to match the easy wins of private schools, be they home or abroad.
One of those factors is that it's a bit harder to deal robustly with parents playing pigeon chess.
But it's not that much harder.
My old school in the U.K. would absolutely have assigned detentions to your children, escalated non attendance to isolation & ultimately fixed term exclusion, & have had zero difficulty in justifying all of the above.
& yes, you would very much, if your posturing here reflected your interactions with your dc's school in reality, have been on the 'hotlist' of parents who teachers were not expected to communicate with directly; you'd be being quietly managed by 'the member of SLT in charge of persistently batshit parents'.
It's quite possible that your dc attend a school where the management is so weak that you do get to behave as you've described. They exist...& they aren't schools that I'd be prepared to have my dc attend, or be willing to teach in.