Get in line, or not

"just belly up to the bar and fight for attention"

But queuing is still taking place at the bar even if you don't see it.

On approaching the bar you will see who is already there and so know approximately where you are in the queue, or at least who is before you. Whilst you are waiting you will see who has arrived at the bar after you.
If bar staff approach you to take your order and it is not your turn the etiquette is to point them towards who you think is next 'I think they were before me'. Once they are served you will likely be served next so won't have to wait long. It's basically good bar kharma.
If you know you have a large and/or complicated drinks order it is also polite to point the bar staff to someone who looks like they aren't ordering much, although this is less common or likely in a busy pub.

If a bar is really busy you will see people leaning forward often with a note in their hand trying to get attention, which some times does work. But if you are too pushy bar staff might leave you waiting for longer.

Queue jumping is a particularly serious crime in the UK, likely to lead to some serious tutting and eye rolling.
The fact that 'young people' now choose to form an actual queue at a bar is probably a sign of the end of British civilisation as we know it.

Emma, what you describe has been my experience at bars stateside. The queue tends to run the length of the bar rather than back into the seating area. Maybe the coffee shop queue concept is leaking its way into pub practices?

ETA: I don’t mean a literal queue along the bar. I mean “gather or cluster” along/in front of the bar.

I was on the London Walks Hidden Pubs tour in June two years ago, and our first stop was a pub tucked into the corner of the Middle Temple (apologies, drawing a blank on the name). The main floor’s a bit snug with little space between tables. So, ye old London Walks folks walk in forming a line at first, getting a feel for the place (you feel on display as it is because it’s obvious you’re on a tour), and we were verbally ripped by a table of three next to us who were lingering over their pints. “Oh, what’s this? Queuing for the toilets now, are we?” I still recall that woman’s voice 2 years later. Once we paid, we took our drinks outside.

I think the point Emma is trying to make is that you don't queue at the pub, either in a line along the bar or back towards the tables. Everyone knows who's next though.

Helen has described the etiquette perfectly. Except that we have just been out for a walk at a local beauty spot in today's gorgeous weather and being a Bank Holiday it was packed. The pub has a small bar so by necessity we all formed a queue!

I have seen people queuing in pubs but normally it is a case of taking your place where there is a gap at the bar. Brits know instantly who was there before them and who arrived after them. If the barstaff get the sequence wrong, it's common for someone to say "he/she's before me", although not everyone does that.
In very busy pubs it can be a case of shouting up when the barman finishes an order. Most of us find that very annoying.
Occasionally, a barman will serve a regular before you, even if he is stood behind you. Again, very annoying.