I camped in Gunnerside Gill a couple of weeks ago -(? probably not far enough from Keld, though.) There's a few flat and sheltered spots.
There are other discreet spots east of Surrender Bridge eg just a bit further than the smelt mill and slightly off-route in a deep valley (the valley West of Surrender Bridge is full of flood debris and is hopeles for camping) and off-route near Grinton Mill (near Reeth) and other spots on the way to Richmond. After Richmond, its a bit more agricultural, and, maybe a bit too military...