It really is as simple as Jumbo Driver wrote above.
ICAO stipulates varying VFR minima depending on altitude etc (quoted by HR200), but they don't say anything about "in sight of surface". You can climb in the clear, fly over an overcast, and land in the clear the other side and be legal.
That's the case in most countries.
The UK adds a restriction to UK PPLs. Unless they have an IR or an IMCR, they must at all times be in sight of the surface. *
So a French PPL with no added ratings can fly VFR on top in the UK, a CAA PPL ditto cannot.
The UK PPL cannot fly VFR on top in France, according to his licence. The eternal question is: if he has an IMCR, can he? (France doesn't recognise the IMCR).
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* Edit: Sorry: the rules changed. In some publications, it's now "with the surface in sight".
Last edited by Keef; 2nd March 2009 at 08:22.