Although these suppliers don't match your search for Riverside venues in Portishead, they are within approximately 40 miles and could still be of interest.
Venues, Riverside venues, Gravesend, Kent
Set on the edge of 3 lakes and woodlands, the Inn on the Lake is a beautiful large venue with panoramic views of the lakes and surrounding woodlands from many event rooms and bedrooms. (Riverside venues in Portishead)