This is strict adherence to the rules of 'legal tender'.
"Legal tender has a very narrow and technical meaning in the settlement of debts. It means that a debtor cannot successfully be sued for non-payment if he pays into court in legal tender. It does not mean that any ordinary transaction has to take place in legal tender or only within the amount denominated by the legislation. Both parties are free to agree to accept any form of payment whether legal tender or otherwise according to their wishes. In order to comply with the very strict rules governing an actual legal tender it is necessary, for example, actually to offer the exact amount due because no change can be demanded."
http://www.royalmint.gov.uk/Corporate/BritishC oinage/LegalTenderGuidelines.aspx
Most businesses (if not unaware of the rules) would choose to complete a transaction using legal tender and proffer change as an act of goodwill. This tends not to alienate customers.