Just did this a little over a week ago from IAD. Used United MP and got UA metal to LHR then Egyptair to DAR via CAI. On the way back we did TK via Istanbul. All flights were great and the Safari was awesome. I can suggest a tour operator who handled everything for our group of 15. We even chartered a plane to get to serengeti since some people didn't want to drive from ARK. Make sure to check our Zanzibar.