Many authority figures have become so fucking spineless in uk now it's ridiculous! Why are they so ineffectual?
If your job requires exerting authority you should have the skills and training to do so and the backing of management if people are arses!
Then such rude, entitled behaviour would happen much less.
I have an independent cinema locally, one of the reasons they're successful despite various multiplexes not too far away is they won't tolerate disruptive behaviour. There's clear signage stating this and it's well known locally. So if customers talk loudly, kick chairs in front etc they're removed. I've never in almost 10 years living here (and it's supposedly a 'rough' area too) heard of anyone daring to kick off if the ushers tell them to go. They're given 1 warning and if the usher has to speak to them again they're gone - no refund.
Another local place is the complete opposite, really lax in dealing with customers bad behaviour and it's lost them a lot of trade. The management that's like this have only been there just under a year, I will not be at all surprised to learn the management are gone or the place gone bust in next few months.
It's also up to us as consumers to complain, we're very bad at this in uk (generally) as we tend to 2 extremes - silently seething but saying nothing to anyone, so they can't rectify, or going way ott and expecting the company we're complaining about to do things that just don't make sense and/or being rude and aggressive.
Other nationalities (generally) take a middle road and complain effectively but politely and assertively, while also being clear if the issue isn't resolved they'll take their custom elsewhere, and as a result customer service is much better.
Op in your position I would contact the company and clearly, calmly explain what happened, what you would have preferred to happen, what resolution you want now.
As a result of being in roles where I've dealt with customer complaints, plus having a dad that has a real skill for doing this, I've learnt over the years to be a really effective complainer - when necessary.
I was really shy and nervous about it when younger, now I don't worry, I make complaints in the manner I described when it's necessary and while I'm persistent and quite tenacious I never so much as raise my voice. But I also don't allow myself to be fobbed off.
It's hard getting the balance right but definitely worth learning.
With the offending cf customers you describe again "peaceful protest" is very effective. I'm a diminutive 5'2" but that actually works in my favour as if they come across as at all aggressive makes them look very unnecessarily aggressive bullies "picking on a wee old wifie" 😂 and most people don't want to be seen that way.
But I also complain to those in charge & expect them to do their job!
My mum - also a "wee old wifie" once stood in the gangway of a train right next to someone who'd taken her reserved seat on a London to glasgow train. She sat my bro right next to him (total pita on long journeys) and soon as the arse needed a pee she took her seat back and refused to move when he came back - she has a cast iron bladder so he couldn't do same to her & of course couldn't complain cos it was her seat!
My aunt once plonked my snotty cold ridden cousin on the lap of a lady that refused to let my aunt use her booked seats while she fetched the guard - this was 70's and guard also took no prisoners and told the lady to move or he'd have the train stopped until she did!
"The problem is, train conductors and venue staff can't physically move people. They can only put hands on a customer if that person needs to be restrained. They can't drag someone out of a seat. Their 'power' is reliant on their patrons following the instructions they give. If do move refuses, there isn't much they can do for fear of bring charged with assault." Sorry but that's no excuse for no action at all! They can assertively and authoritatively ask them to leave, they can refuse eg to start the film until they do, refuse them as customers in the future and tell them so - there's lots they can do that doesn't require physically removing them!
Certainly where I am train conductors & such won't hesitate to get police involved the SECOND some arse tips into any kind of aggressive or antisocial behaviour. It's one of the things I really loved when I first moved back to Scotland. I was on a bus not long after I moved back and a guy JUST missed it, the driver had closed the doors and was slowly leaving the stop. Guy ran in front of his and punched the windscreen to try and get driver to stop and let him on. Driver refused, alerted bus company with description - which basically meant the guy wasn't getting on ANY bus that day! I asked my parents & aunt what would happen if he kicked off with other drivers and they told me the bus company wouldn't hesitate to report him to police and that police here take a very dim view of such behaviour and would have spoken to him warning him he cannot behave like that and what could happen if he did again. Not long after I read a local paper article where a (different - clear from description) iirc guy was barred from using the local buses for 6 months.
If those with the authority to deal with such people do so, the behaviour is less likely.
"See, that might work and then again it might not.. and if it doesn’t (assuming no aggro from the seat squatter), you’re just likely to piss off other people just going about their business in their allocated seats" if I'm a customer in my allocated seat witnessing/being affected by someone else trying to access THEIR allocated seats the person I'm gonna be pissed off with is the arse who's sitting in a seat that isn't theirs, that they have no right to and who is blocking it from the rightful occupant! Not the poor dude, who possibly paid extra to reserve, trying to get what they booked!