My impression is that it's due to the same reason Americans leave large tips if they can. Without an established class system, money is the signal used to demon

My impression is that it's due to the same reason Americans leave large tips if they can. Without an established class system, money is the signal used to demonstrate your standing in relation to others. Incidentally, money and class are, in my experience, the most misunderstood aspect of the English class system by Americans. There is a loose correlation, but it's only very loose. It's quite common in the UK to have upper class families with hardly any money beyond inheritance, and conversely rich lower class families who might run their own businesses. Interestingly, the upper class man wouldn't feel the slightest bit inferior for neglecting to buy a diamond ring for his fiancé unless doing so was the "done thing" by previous generations of his or her family, whereas the rich lower class man would be clamouring for the biggest one he could afford. Generalizations, but so termed due to being generally true.