I live in Ireland but fly home a few times a year to the D.C. area. A few suggestions:
Open a BritishAir.ie or .co.uk or some other country's page to get different pricing!
Try all three DC airports as you do pricing.
And know this: after years of this regular route going often, i will ONLY fly BritishAir out of BWI now. I've had the best service. They don't charge for alcohol (this matters to me) and the food and movies are great. They rebooked a flight for me at no charge when i broke my arm in another state and couldnt get to BWI in time. It's BWI to Heathrow and since its night you can sleep on the plane and get there in the morning ready to explore!
I'll NEVER fly United again. On a return to BWI they cancelled, 9 hours early, a 5 pm NJ to BWI flight "due to weather" on a glorious day - so of course we had no recourse as it was weather related. They had no flight til the next day. We ended up taking the train which United ticketed incorrectly and the Amtrak people tried to kick about 35 of us out at Baltimore train station since the train tickets weren't all the way to the airport. The guy finally let everyone stay on to the airportbecause people were freaking out. A nightmare. United never apologized or even acknowledged our complaints!