Oh no! What? Rain in April in England? I can hardly believe it!
But I wouldn't worry, because as long as it is not blowing a gale at the same time, it will make it easier because it will keep us from getting too hot. I agree with everything tricialitt says.
Plus when it rains there is more chance of chafing, so need to prepare for that.
I find it easiest and most comfortable to race in the lightest possible clothing, and if it rains, the only change I might make is to take a light, foldable cap to keep the rain off the top of my head and out of my eyes, to be removed when the rain stops after 3 miles, and I get too hot!
Good luck.