Vis minimum for SVFR depends on the country, & sometimes the airfield within the country. However, the way the UK use it, a SVFR for a normal PPL needs 10K
Worth checking out 14 and 15 on
http://www.higherplane.flyer.co.uk/faq.htm
which seem to cover it.
So many normal PPLs get confused about svfr because they hear IMCers talking about getting in and out of Controlled Airspace with poorer visibility and they don't realise they can't use it like that.