Challenge 25 is a scheme that encourages anyone who is over 18 but looks under 25 to carry acceptable ID when they want to buy alcohol. Challenge 25 builds on the Challenge 21 campaign introduced by the British Beer and Pub Association in 2005, who represent the beer and pub sector. It’s now run by the Retail of Alcohol Standards Group, which represents alcohol retailers.
So, if you are lucky enough to look under 25 you will be asked to prove that you are aged 18 or over when you receive your delivery. If you are under 18 you are committing an offence if you attempt to buy alcohol.
For more about the laws on alcohol visit the 'Licensing Act 2003' government page.