As PPLs we have been taught never to go closer than 10 nm to an active CB cloud
This is the safe option that flying schools must say to new PPLs.
Personally unless they look very bad I just try to not actually go through them. In my experience the average UK storm is not very dangerous ... of course there are non average ones.
I have met some more interesting CBs in West Africa and USA .... I definitely would not want to get inside them .... but even they have to be very big to make me keep 10 miles away.
Just my personal take ... maybe I have just been lucky.