Megan's Law. Nevertheless, there is an expanding Child Sex Offender Disclosure Scheme described on a government website here. The scheme commenced in September 2008 in 4 Police Areas and will, from August 2010, be extended to a further 18 with a complete national "roll out" planned by March 2011. There is also a Child Exploitation and On-Line Protection Centre. In essence the scheme enables certain persons with concerns about an individual to have checks made on that individual. It seems that, in the 4 pilot areas, fathers of children have been one of main users of the scheme as they seek information about their ex-partner's new boyfriend - see The Guardian 3rd March. The government and Police have claimed that the scheme is proving to be successful - see Home Office 3rd March 2010. Only time will tell whether a disclosure leads to some form of action by way of revenge.
Note: Sexual Offences Prevention Orders are made by the courts under the Sexual Offences Act 2003 s.104.