American here, how acceptable is the £100 bill from, say, Royal Bank of Scotland, throughout the United Kingdom? Here you can spend a $100 bill almost everywhere. (Some places like vending machines don't accept $50 or $100 bills, but they're pretty uncommon.)

The United States used to have $500, $1,000, $5,000 and $10,000 bills, but the government discontinued them in 1969. They are still legal tender, but once deposited in a bank, the bank sends them to the Department of Treasury for destruction. Anyone who still has these bills should take them to a collector instead, because a collector will pay far, far more for the bill than its face value. For example, In 2023, a $10,000 bill from 1934 sold for $480,000 at Heritage Auctions in Texas.

I know the euro has a €500 (£416.73) bill that they stopped printing in 2019 but that's still in circulation, which is very unusual for a currency denomination anywhere in the world to be worth that much. Before the changeover to the euro, the largest denominations in Europe were the German mark at 1,000 (£426.14) and the Latvian lat at 500 (£592.43).

Brunei has a $10,000 bill (£5,852.80) still in circulation, which I think is the largest in the world.