To me, April in Cornwall is about spring flowers.
First the wild ones. These are a bluebell wood near here, gorse on the Cornish Coast Path and blackthorn blossom. There are blackthorn hedges on the top of the Cornish stone walls. There are also wild daffodils and primroses everywhere.
Then there are camellias, rhododendrons and magnolias in profusion in gardens both private and those you can visit.