Meaning of PROB30/PROB40 in England
When I was in flight training I asked an instructor about this and another instructor said his friend was doing aviation meteorology in the area.
According to him, his Met friend said that PROB40 means there is some weather there, a big front or something, and they don't know if it will move over and get you or whether it will stay where it is.
PROB30 is when the landscape is expected to be peppered (or polka dotted) with weather. All these things will be drifting by, but you don't know whether one of them is going to target directly through your aerodrome.