Normally, I don't have to worry about such things. I go to the pub mainly on Sunday and Monday afternoons. But on the few times I've actually been to a place with actual people in, this is what I've worked out about finding your own personal drinking spot.
(1) If you are a lone drinker, try to find the table/spot with the fewest seats. While it's rare for anything to be said, you may get a lot of sideways glances if you're occupying the only table-for-four when two couples come in together.
(2) If all seats are taken, then you can stand up at the bar (or pull up a chair if one is available). Though it's best not to engage the staff in too much conversation if you don't know them. Best not to be seen as an inappropriate bore in a strange pub.
(3) And if you do sit at the bar, don't order food and eat it there. Nobody wants to hear someone chomping away while waiting for their pint to be pulled.
(4) If there is an empty seat at your table, try not to dump your stuff on it. Someone may require it (see above).
(5) If in a large group, try not to disrupt the pub's seating plan too much by moving tables together and purloining nearby chairs. It will not look good, especially if you all are only in there for one drink.
(6) If no seats are available and the bar is full, it's best to leave. Or have a swift half. and then leave.
I'm sure there are many others I've not yet experienced. As in many things with human interaction, you only figure out something when you see it being done wrong.