The idea is that if you get a new job, before you leave your old one you must organize a pub event and pick up the tab for everyone's drinks.
It's a decent idea; you have the chance to say a proper goodbye to your coworkers before you move on, and it's an informal event in a casual setting.
There's another tradition when it comes to leaving a workplace. Coworkers not only organize a card to pass around, but they also put money into a collection which is used to buy a gift for the person leaving. That person is then presented with the gift in the office, as their manager says a few lighthearted words about their contributions and wishes them well.
When I left AIG back in October, I was presented with a "sorry you're leaving" card - on the front, a double-decker bus speeding off - and as a gift a bottle of eighteen-year aged scotch. That evening I hosted my leaving drinks at a pub called Jamie's near St Pauls. I enjoyed the whole experience, it provided a degree of closure as that chapter of my career came to an end.
Moreover, it is quite common in the workplace (and almost expected) to mark all sorts of occasions with food or drinks. People bring in food all the time; many mornings start with an email to the tune of "I brought in cakes for my birthday, please help yourself!" Not just birthdays though; when you're out of the office traveling for more than a couple days, it's an unwritten rule that when you return you're to bring in treats from your trip. One time someone even brought in food to celebrate their cousin's graduation. I thought that was a bit of a stretch.
I've grown quite fond of these occasional morning pick me ups, and I'm sure I'll miss them when I'm working in the US again. Perhaps it's a concept I'll have to bring back with me.