I'd use beta.itasoftware.com.

in the from place a semi colon list of the airports that you might want to fly out of...

something like this.

from: BRS;LON::IAD UA+ (bristol or london area airports through IAD using United Airlines or it's partners.. as far as I am aware that is what the UA+ does - although it doesn't typically show any of the more "obscure" partners like croatia airline).

then do something similar for the entry airports.
to: IAD;CHO

in any light, i'm hardly an expert on the syntax for this booking engine, but a quick browse through the mileage run forum would probably net you more information about this than you could possibly care to know.

also, CHO is probably one of the most expensive airports that I have ever flown into. if you find that you have extra time, you're probably better served by renting a car out of dulles and driving down.