The privileges of your licence (ANO Schedule 8) state that the minimum visibility for a PPL is 3Km. If you have a valid IMC rating, that becomes 1800 metres below cloud. These are the legal minima and form part of the licence privileges. The ANO can be found at:
http://www.hmso.gov.uk/si/si2000/20001562.htm
In addition, there are the VFR minima for different Classes of airspace which can be found in the AIP ENR 1.2. For Class G airspace below F100 the visibility required is 5 Km however, for aircraft operating below 3000 feet at 140 Kts or less this reduces to 1500 metres, clear of cloud and in sight of the surface. The AIP can be found at:
http://www.ais.org.uk
Your PPL restricts you to 3 Km so you cannot take advantage of all of this reduction. A CPL holder can.
If you obtain a copy of LASORS there is a guide to UK VFR rules and procudures at the back.
http://www.caa.co.uk/docs/33/LASORS.PDF