david viewing
Sorry - didn't read your post carefully enough.
An unrestricted FAA PPL does NOT allow you to fly VFR on top. For that, you need an IR. That very question came up, both in my FAA IR written, and in the oral during the checkride. A "VFR on top" clearance is an IFR clearance, but without all the restrictions that apply to flight in IMC.
The way the US "reciprocal" PPL works is that you are governed by the more restrictive of the FAA rules or the rules that apply to your home licence. That means, mostly, that the CAA rules are the definitive ones.
My understanding of "VFR on top" is that you can't see underneath from where you are now, and couldn't get back down there in VMC right now. There's bound to be a far better definition in one of the books!