When thunderstorm activity occurs in the UK, it tends to take the form of quite specific individual cells. Whole lines of storm cells of the sort seen in the US at certain times of year are far less common.
So, with this in mind, normally, when a storm arrives at your airfield, sit it out for 30-40 minutes and you will find that it clears and you are good to go again. Just occasionally, you are unlucky and it clears in the direction of your destination.
PROB30 is often associated with TSGR and similar because since by their nature the storms are specific individual cells. The forecaster cannot be sure that any cell will pass over any particular airfield, even though he can be pretty sure that there will be storms over a wide area.
So, when you see PROB30 TS, assume that you will be flying in or around storms with some certainty. You just can't be sure that one will pass over your airfield.
When you see TEMPO 1015 TS you can assume that the storms are rather closer together, since in this case multiple storms are predicted to pass over the station in the TEMPO interval.
Hope this helps