If you are below 3000ft and flying at less than 140kts, you need a flight viz of 1500m or more.
3000m for a UK PPL (no IMC or IR) at all times.
My own personal interpretation of the VFR rules is that you can fly above clouds providing that if the donk stops, you can get to the ground without losing sight of it.
I think this is debatable because it would be an emergency, and in those there is no need to be legal. A bare ICAO PPL doesn't prohibit flight above an 8/8 overcast layer. The need to be in sight of the surface is an additional requirement imposed by the UK CAA, and a few other countries.