It may be an offence under The Animal Welfare Act 2006 to

 release foxes once trapped.

For more information, please consult The Animal Welfare Act 2006

http://www.opsi.gov.uk/ACTS/acts2006/pdf/ukpga_20060045_en.pdf

 Below is a link to Natural England's Policy on Urban Foxes:

http://naturalengland.etraderstores.com/NaturalEnglandShop/SIN003