Sometime before the 12 hour period, one must make a choice whether to seek care or not.
Fortunately the previous tweet linked to the original source, a published study of 5 million ER visits which appears to have a full set of controls and specifically measures time to inpatient transfer in excess of 5 hours as a mortality risk factor https://emj.bmj.com/content/emermed/39/3/168.full.pdf
And since we're talking specifically about the subset of NHS emergency department attendees who are more likely to be dead within 30 days if it takes longer than the NHS' target of four hours for them to receive an inpatient bed, I'm pretty unconvinced that the alternative route of booking a doctor's appointment in the next few days to see if he'll refer in a week or two is the more appropriate one...
If you think about it, this makes a lot of sense because ERs are tuned for low latency, low throughput, high cost for emergencies, and if lots of people who just need urgent care go, they make it hard for the emergencies to be seen with low latency.
Anyway, so I went to the ER and to begin I did sit in a chair like you said, however I don't think anyone was keeping an eye on the patients, since by the time someone came to look at me, I had been writhing in pain on the floor for an hour or so, and it had taken another patient to call a nurse over.
All and all it took 4 hours before I got to see a doctor, but this was pre-pandemic.
Still, I think you're probably right and people do show up to A and E for non-urgent issues, however it's probably also tied to how difficult it is to see a GP these days.
For the gallbladder, if it's just an infection, not fatal. If it's a gallbladder attack, it can lead to rupture. Definitely needs emergency surgery.
During my visit to urgent care (so early in the days of COVID that there was sign saying only people who had just flew in from Wuhan could be tested), I was seen immediately, the doctor stopped while seeing me, said "the person in the next room is having a heart attack and I have to send him to the ER immediately- I'll be back".
FWIW, this isn't always a call that person makes - I've had multiple family members advised to attend by the non-emergency service (111) and ended up with a 6-8 hour wait.
You also can face lengthy waits for something that needs treatment soon but isn't immediately life-threatening (grandfather fell and hit his head on a table, requiring stitches - ~4 hour wait for a handover from the ambulance, another 2-3 before he received an MRI and stitches).
Or the ER was crap. Like the time I waited 12 hours (because I had no choice, I needed a surgeon) to have my foot put back together.