Glastonbury Abbey, Somerset. The usual foundation gate given is 710, but archaeological evidence predates this to the early 7^th century. there are stories and legends about earlier history; the notion that it was founded on the 1^st century by Joseph and Arimathea is rather fanciful, sadly. Interestingly, recent excavations at the Beckery chapel nearby suggest monastic activity going back to the early fifth century, making that the oldest monastic site in England.
  After 1066 the Normans took it over, and it continued to thrive, until1184 when a great fire destroyed parts of the abbey. Money was needed to rebuild it and, very conveniently, the grave of King Arthur and Guinevere was discovered in 1191, which brought in the pilgrims. It became one of the wealthiest monasteries in England, which made it an important target at the dissolution of the monasteries. Poor Abbot Whiting refused to go along with it, and he ended up being hung, drawn and quartered on Glastonbury Tor.