Hi,

CO should be able to tell you the min. connecting time domestic to international. They may actually have the IAD-Intl. in their computer as it involves a termainl change.

You will arrive in Terminal A, in the middle by the A20's gate. You will then have to enter the non security area and get on the monorail from Terminal A to Terminal C. The monorails run every feww minutes. At Terminal C you will have to go thru security all over again.

Yes, you'll be able to check your luggage all the way through to Gatwick.
Cheers