As a paddler with a flatwater background you will have very strong skills in forward paddle technique. The wildwater racing boat requires a little adaptation from the pure flatwater technique - but overall it is very similar.
As a paddler without WW skills, getting an understanding of how rivers work will be key. Have a go in a wildwater racer on flatwater to see how you like it, and to understand how to steer the boat in easy water.
You can have a go on some very easy whitewater, and the BCU star awards will help a lot to assist you in getting a better understanding of rivers.