Not strictly speaking illegal - if the cameras are there for a specific purpose (in this case to catch antisocial dog owners in the act), the signage states that they're for "public protection and crime prevention (dog fouling is a criminal offence under the "Clean Neighbourhoods and Environment Act 2005") and the "data controller" (in this case the householder) allows "reasonable access" to any recordings and only retains said recordings for a "reasonable time" (this is usually a maximum of 31 days unless the footage is to be used as evidence for a prosecution).
I'm a CCTV engineer and am involved in this sort of thing on a daily basis.
A summary of the relevant legislation (Data Protection Act and Article 8 of the European convention on human rights) can be found here: http://www.guardian.co.uk/commentisfree ... ghway-film
============= Sig =============
Just a y2k FSR with some modern bits