2 points.
Firstly, nearly all rescues in the Severn, and certainly above the confluence of the Wye, are dealt with by SEVERN AREA RESCUE ASSOC, who are a voluntary body like the RNLI. RNLI do not go up that far, as their nearest boats are either at Cardiff or Portishead(?). They admit the the Severn is a special case due to it's continually changing course, and they prefer to leave it to those who know the river. SARA are very efficient and do an excellent job FOR NOTHING. They are fully supported in their work by the RNLI.
Secondly, the tide can completely alter the course of the navigation channel above the old Severn Bridge at each tide. The Pilots know the signs and can "read" the river, so they know where they can and can't go. Even Captains of the few cargo vessels that use the Severn regularly ALWAYS use a pilot.