@DiapDealer
I am very pleased to report full success of your Regex ( I used the first one) which deleted successively in seven busy rounds: 53/22/7/5/2/2/2 occurrences of the <br /> tag.
This is only the top of the iceberg, because on the odt I previously manually destroyed probably about over one hundred. I did not know then I would use your regex.
For information, this is the styles break-up of the test EPUB (classes only):