Rule 114 of the Highway Code says you must not "use any lights in a way which would dazzle or cause discomfort to other road users, including pedestrians, cyclists and horse riders" but I think, as this is enshrined in law (RVLR reg 27), it refers to headlights, fog lights etc. If they were talking about brake lights then they would be specified, as are rear fog lamps and reversing lamps. Not just lumped in under Item 11, Any other lamp.
I have never been dazzles by brake lights, not even the high level ones that stretch right across the top of the rear screen, and I have a cataract in one eye. Red light does not destroy night vision. Rhodopsin in the human rods is insensitive to the longer red wavelengths, so traditionally many people use red light to help preserve night vision. Red light only slowly depletes the rhodopsin stores in the rods, and instead is viewed by the red sensitive cone cells. That is the reason the bridge of a ship or the flight deck of an aircraft is illuminated by red lights.