Love fish sandwiches! That place in Colonial Beach sounds great. A bit out of the way for me, but might get the wife interested in a day trip to Northern Neck this fall and get a fish sandwich for lunch.
There is (was) a local vendor called "Southern Fish Fry" that used to come to the Herndon Festival and sold fish sandwiches similer to the ones mentioned here. While most festival goers would be enjoying their "chicken on a stick", "pseudo" cheese steaks, etc., I would sit down to fried white fish between two slices of white bread, a dash of hot sauce and a cold beer!