There is a funny little one called the Sheringham (Norfolk) Pottie Festival, first weekend of July. It's Morris dancing and free-for-all singing/playing in the evenings. Lovely setting, countryside and shore. Shantymen's choir. Camping available. Also the Worstead Festival, also small but beautiful setting, near The Broads, on for three days, 29 30 31 July. Both these events have VERY friendly and welcoming clientele, and if the weather is nice, they can be idyllic. But they're not sophisticated or mind-blowing, just gentle country affairs.
(PS for Americans, The Broads are not what you may think, they're man-made lakes!)