I'm still not 100% sure how big Brigit was in what is now England. The odds are high, but there is nowhere near the evidence available that exists in Ireland for her mass veneration - as St Bridget she is second only to Mary and matches Patrick for sites devoted to her. In Kildare (Kil D'ara - The Church of the Oak) she had an eternal flame mantained by nuns until William the Conquerers men destroyed it.
Britton was the word the R*mans used to describe the people of that spoke what are now called the Brythonical languages Cornish, Welsh and Breton. Why they chose that is anyone's guess. It could just be the tribal name of the north French Gauls they first encountered. These could have been the people of Bree or Bri, an ancient Goddess with Brigit connections.