A number of my family, including myself, have been back and fourth to DC over the last few years as some of my relatives were posted out there. Collectively we've flown BA, Virgin and United...
United - family was far from impressed with the service...
Virgin - most of the family used them. They fly into Dulles though - great service, great prices (comparable to BA, though sometimes a bit more expensive)
BA - they are the only airline that fly direct from London to BWI. Service is usually excellent, prices are dirt cheap (£378.50 rtn at start of October at the mo) and they have great regional connections from as little as £50 one way. Terminal 5 is now running smoothly and so you shouldn't have many, if any, problems.
The other airlines connect in Philadelphia, Chicago and Newark to name a few places, however if you search the forums you will find that transiting through the States is best avoided at all costs so I'd say go with BA
Hope this helps!
