Run a book is a sweepstake on the length of time. You put a £ into a glass and guess how long it will be to the nearest 5 seconds. The person closest gets the cash (and is then forced to spend it at the bar normally).
I know of another set of customs (which might be a particular UK Army Regiment - Mr E, have you heard of this?) where you're supposed to drink certain drinks at different times. For example:
Between 09-11: Gin & Tonic 11-13: Wine 13-17: Pimms 17-19: Wine 19-02: Beer (or anything in a glass)
I think this might be generalised to food or drink for some comedy effect. "You're eating CHIPS? But it's 14:30!"
I like that one a lot. I really don't hang out in bars but from what I understand hanging out at the Pub is something entirely different in the UK than in the US. That in itself is an interesting cultural difference. In the US going to a bar is a weekend thing mostly for single people or people trying to escape home life.
Pub is both a family place and also a place for people to escape the family. Every pub has its own feel. In Scotland, people often go to pubs as a good place to start a fight.
Common gestures are always a good source of fun (and potential conflict!). When I lived in the Philippines we were told never to make the familiar Western "come hither" gesture to Filipinos, as they considered it equivalent to a gesture that we make with the middle finger. I was also told that, for the same reason, one shouldn't hitchhike in parts of Africa. The Vulcans of Star Trek (who are touch telepaths) don't shake hands, and hugs are out of the question. In Jack Vance's Planet of Adventure series the color orange was the sign for sexual entertainment, and any woman wearing orange was assumed to be a prostitute.
There are lots of ways to play with this. Perhaps there is a culture in which a thorough pat-down is the traditional greeting -- evolved, perhaps, from a search to prove that one had no concealed weapons -- and in which refusing such a greeting signals hostile intent. Or maybe the people of the nation/planet where the PCs find themselves greet everyone with a hug and a kiss on the mouth -- but make sure that it's an A-frame hug, because an embrace with hip contact is a sexual proposition (or even better, a marriage proposal). If you really want to screw the humans over, send them to a world where the natives have four arms and certain crimes are punished by cutting an arm off; those beings would tend to see all two-armers as two-time convicts. Bonus points if the penalty for a third infraction is beheading. };)