With a vanilla PPL you can not fly other than in direct view of the surface. Full stop.
If such is allowed on a FAA PPL in the USA (which I doubt but will look in the FAR/AIM as soon as I get home) you would still not be allowed to do this in the UK, just as a French PPL would not be allowed to do it here or a UK PPL over in France.
You will either need an IMC rating in the UK or an IR elswhere.
I don't think there is a sharp definition with regard to the amount of cloudcover you can fly over and still claim to be in sight of the surface.
As with so many things, there is no policeman on the corner of every cloud, but be prepared to have your case ready if things go to pot.
FD