DFC,
Have a good look at;
http://www.caa.co.uk/docs/64/ATS_Classifications.pdf
Note, the visibility minima is horizontal not vertical, so you could be at 2999 ft, in sight of one blade of grass and have a horizontal vis of 50km. This is LEGAL, I make on comment on the practicalities. Remember the original question was about flying above some cloud, not a discussion on horizontal vis.
Rod1