yay! I'm hoping that SFO-LHR-IAD doesn't mean connection in LHR but rather that you wanted to go there for something.
Anyway, on transcontinental UA the food isn't great but it's not bad if you're not too sophisticated for the flight. I'm sure on C Int'l you'll be fine. Just be sure to double check before the flight that they actually put in the veg order as on one segment (LAX-MCO) they couldn't find my name on the veg list and just said "don't eat the bacon that comes with breakfast". Hehe.