Echo the "don't fly through it if you can't see through it" advice. Suffering as I do from back trouble (it has a large yellow streak down it) I wouldn't be flying in anything more than isolated showers. I have gone round in circles south of Compton Abbas waiting for a shower to go through. This started to make me dizzy but reversing the direction did the trick.
The problem with widespread rain is that it's usually accompanied by a lowering cloudbase and there's no guarantee that it won't intensify to the point where you can't see through it.
Mike