Data on the number of registered sexual offenders in England and Wales is available in the annual published multi agency public protection arrangements (MAPPA) reports:
http://www.probation.justice.gov.uk/output/page30.asp
Each police force monitors all registered sex offenders in their area and knows which of these offenders is currently compliant with the notification requirements. Offenders in breach of their notification requirements have committed a criminal offence and are liable to a maximum penalty of five years’ imprisonment.