'Few things are more pleasant than a village graced with a good church, a good priest and a good pub.'
Observe and respect the atmosphere you find in a pub. If your fellow drinkers are sitting alone or in couples quietly chatting and reading papers don't ruin the mood with loud banter or uproarious games of darts and bar billiards. Likewise, if you are in a busy town-centre pub after work on a Friday there's no point complaining about the rowdy group of office-workers letting their hair down at the table next to you.
Pubs are very sociable places so be prepared to exchange small talk with strangers. On the other hand, if a solitary drinker is lost in contemplation of their pint or crossword, respect their privacy - if they want a chat they'll make it obvious.
If a group of you are drinking together it is usual for people to take it in turns to buy a round. Don't opt out of rounds, or hang back; you shouldn't have to be asked.
Don't hog a space at the bar, blocking the way for other punters. Pubs are usually liberally supplied with chairs and tables, so try and move away from the bar when you've ordered your drink.
Be aware of 'regulars'. These are habitual customers of the pub, who might have their own favoured seat, special glass, or repartee with the barmaid. The pub is their second home, so respect their prior claims.
Tipping is not necessary in a pub. However, if you feel you've received particularly notable service you may like to offer the bar staff a drink.