The law was changed in 2013 to clarify the rights of online sellers and buyers.
They can be read in full here: https://www.gov.uk/accepting-returns-and-giving-refunds
Here is a brief description from Gov.uk:
Online, mail and phone order sales
Online, mail and telephone order customers have the right to cancel for a limited time even if the goods aren’t faulty. Sales of this kind are known as ‘distance selling’.
You must offer a refund to customers if they return goods within 14 days of receiving them.
You must then refund the customer within 14 days of receiving the goods back. They don’t have to provide a reason.