I'm afraid you're incorrect here - the queue is simply memorised by all the people who go to the bar. Most of the time the queue is held in the bartender's head. If you see the bartender say 'who's next please?' then they've lost their place - the queue becomes apparent because someone will point at the next person. Everyone knows exactly who was before them.

If you accidentally cut in or accept the bartender's attention before someone who is waiting, you're still glared at and must apologise. It's more prevalent outside of London than within the M25 - but even when we appear to be a scrambled mess, we're still a queue - and now you know why people might not have been as nice to you in bars as the post office. ;o)