”If a pilot is suitably qualified (IMCR/IR) is it legal to fly in VMC without sight of the ground?”
The permit restricts you to VFR. If your licence says you are VFR above cloud then you are VFR above cloud.
”Also, as I understand it you can't fly over built-up areas, right? Is this regardless of your altitude and regardless of glide clear ability?”
This restriction was removed a few days ago.
”And what about on the near continent? e.g. France.”
A Permit aircraft is restricted to UK airspace unless it has the permission of another state to fly there. A standing agreement exists throughout almost all of Europe. The exceptions from memory are Belgium, Spain and the Channel Islands. All of these will give you permission if you apply in writing, the rest, you just go.
Rod1