The banknote will enter circulation on 23 June – Alan Turing’s birthday – and will be made of polymer.
Source: New Alan Turing £50 note design is revealed – BBC News
Not that many people will ever actually SEE a £50 note. They’re rare in circulation and the joke is that only drug dealers and money launderers will see them. I’ve only seen them a few times myself in 20 years of travel (it’s like paying with a $100 bill in the USA).