Oh dear.... a plain PPL cannot fly in sub-VMC conditions, as a PIC, under any circumstances whatsoever.
I think the simple answer which the original poster is looking for is this:
SVFR is a weird "legal device" for enabling a plain PPL holder to fly in Class A airspace.
SVFR is possible only if the Class A extends all the way down to ground.
The practical use of SVFR is for transiting over Class A airports (in the UK this basically means Heathrow) or for landing at Class A airports (in the UK, for little planes, this basically means the Channel Islands).