There are two issues here, the privileges of a PPL are contained in the Air Navigation Order Schedule 8 and limit the holder to 3K minimum vis unless they hold an Instrument or IMC rating.
The VFR minima for UK airspace is given in the UK AIP
http://www.ais.org.uk/VFR/VFR.PDF
You must take the most restrictive minima for VFR flight. If you hold a UK PPL with no instrument qualification, it is quite legal to fly IFR outside controlled airspace provided you do not exceed the Schedule 8 Legal Minima of 3K vis, and if below 150 Kts, clear of cloud and insight of the surface.
[This message has been edited by Noggin (edited 07 July 2001).]