There are rarely change machines near (food) vending machines in the UK. If you don't have coins, then you're out of luck. However, in amusement arcades (places with fruit machines, video games etc) you will often find change machines. Sometimes you find them at railway stations too - British railway stations now charge you to use the public toilet. It's usually about 20p-30p and you must have the correct change for the machine. Having a note is impossible but if you only have, for example a £1 coin, you won't get any change from the machine at the entry point. You can, however, put your £1 coin into the change machine nearby and receive five 20-pence pieces, one of which will grant you entry to the public toilets.