£129 for the trackday (plus van hire,fuel, consumables, and a B&B the night before)
It's an outdoor activity, so bad weather is just a risk you have to live with. The trackday organiser won't give you a credit note or refund just because you're worried about getting damp. I have had days cancelled before, but only because the circuit was covered in several inches of snow, but I've also done days where it's snowing while I'm out on track.
The best way to get confident in the rain is to ride in the rain. There's no substitute. You need the right rubber though - score a spare set of wheels and some race wets, and you'll be amazed what you can do. Grip-wise, it's almost like riding in the dry. Right up to the point you fall off, anyway...
http://youtu.be/jzNTNCTADWA?t=6m47s