Some more composers to join the "Classical" list:

George Butterworth
Percy Grainger
Gerald Finzi

Perhaps if we're referring to tradition English folk music we should think about the many people who helped to collect and collate the vast amount of material which we have - besides those who have already been mentioned as composers or performers:

Sharp
Child
Karpeles
Kennedy
Gardner
Hammond
Baring-Gould
Broadwood
etc