Hi Clarkee
I did my CBT a couple of weeks ago and although it wasn't raining it was very gusty and I got battered by the wind! It didn't matter though as I was buzzing and enjoying every minute of it.
Just make sure you don't go too fast into corners and make sure you avoid any painted lines/arrows on the road around the bends.
Your instructor will advise you on what to do exactly in the rain and will give you a chance to practice away from the busy roads before doing your minimum 2 hours ride.
Trust me, whatever the weather you will have a great big smile on your face at the end of the CBT!
Have fun
Kenny