As with most such official documents [0] a UK birth certificate bears a Crown Copyright [1] imprimatur. Therefore, the thinking goes, your actual name is owned by the state and not you. Therefore using it is a fraud, therefore you don't need to pay bills/debts/fines etc, because mumble mumble reasons. It's a tradition or an old charter or something.
[0] And OS maps and geodata.
[1] http://www.nationalarchives.gov.uk/information-management/re...