That used to be more of a problem. prior to the Royal mint resizing the 5, 10 and 50 pence coins, the 10 and 50 were exactly the same size as other international coins of a much lower face value. The Kenyan 1 and 2 Shilling comes to mind. A friend at University from Kenya used to use those coins to phone home in payphones. I don't know the specific exchange rate at the time, but they were worth pennies in Sterling from what I remember. The Irish coins, worth less than UK Sterling, also had similar sizes and some of the Australian and New Zealand too. Usually the coins were small face value, but even so, it was possible to pass them off.
Now it works the other way round (the 10 pence is pretty much the same size as a US quarter). Not much of a saving (25¢ is around 15p), but I used to use them from time to time in college while I was doing my laundry.
