Ummm, maybe I'm wrong, or maybe I'm just picky:
As soon as you're no longer "in sight of the ground" you have to be IFR. So there's no such thing as VFR on top.
Sorry Welliewanger, but you're wrong (at least as far as the UK is concerend). You don't have to be in sight of the surface to be VFR - if you're 1000' vertically, or 1500 meters horizontally clear of cloud then you are also VFR if you meet the visibility requirements (5km below FL100, 8Km above). check the
CAA VFR Guide
However, the UK issued PPL has a restriction which requires you to be in sight of the surface, unless you hold an IMC or IR rating.
In response to Chuck's original question - yes I have done so in the past, but as Teddy said I'd want a reasonable cloudbase.
Brooklands